I have not done any master set but would love to learn more about it.
what is a master set? What should or should not be included besides your Card List from the Pokemon website? Is this a personal preference? Did collectors just made up a ‘master list’ or its an official thing? And then theres ‘a true master set’? What if there are promos exlusive to that country only?
In general, a ‘master set’ is usually one of two main categories:
(Keep in mind that with option 2, a new related card may release years after the initial set. The most extreme example I’ve seen of this is the Build-a-Bear Snorlax of the XY Flashfire set. The set itself was released on May 7, 2014, yet this Snorlax was released in August/September 2019, more than five years later.
But even though those are the main two categories, almost everything in between (or even beyond - e.g. unreleased prototype cards, repeated misprints, different languages) would be considered some kind of master set as well. The fact that you had to ask this question (and it has been asked loads of times in the past - or minor variants of that question: June 2015; Nov. 2017; Nov. 2019; Jan. 2020; Oct. 2020; June 2024), says enough imho. There simply isn’t a clear definition for what a master set should consist of. 
It also depends a lot on the set itself (e.g. EX era codes or the a vs b cards of Aquapolis to give some random examples of weird shenanigans in sets).
I personally think @genosha worded it pretty good, for master set collectors, it all comes down to budget:
As for your question:
I’m pretty sure someone once came up with the term, and it just stuck. It happens with a lot of Pokémon-related terms, like Shadowless, No Rarity, etc. A lot of terms come from the community and were just repeated enough to became a semi-official term. 
Greetz,
Quuador
I remember at some point there were collectors arguing a master set isn’t a master set if you don’t have 4 copies of each card (because you can only play 4 copies of a card in a deck).
And that is a totally fine way to collect for someone who has the ability and budget to do so.
But in reality, anyone can make rules for their own type of master set and that’s totally fine. I think the baseline for a master set is one copy of all regular cards and one copy of every reverse as Quuador mentioned. After that it’s users discretion.
It’s a made up term so there’s no strict definition.
In my opinion, a set is a master set if you can’t get some nerd to say “ackually…” when you show them the set.
Going out of order for a minute with your questions;
“ Did collectors just made up a ‘master list’ or its an official thing?”
Pretty much. Pokemon games have master balls, why not make it part of collecting to have a master set. People like to think they are better or more special than others.
“ Is this a personal preference? ”
Absolutely and no one agrees. For me a “master set” is defined by it being a “master” with the definition of being the highest quality made of which other sets would be made from.
“ What if there are promos exlusive to that country only?”
Depends on your collection but if you are claiming to have all languages in your collection then you’d probably include that.
“ And then theres ‘a true master set’?”
Well this is against what I said 2 questions ago but there are a limited amount of some cards, youd have in some cases 8-15 master sets in the world when you include some limited promos. So exceptions have to be made to allow it to be reasonably attainable with time and dedication. I would classify “true master sets” to include trophy and cards that are limited in promotion and amount.
“ What should or should not be included besides your Card List from the Pokemon website?”
Still subjective but generally: variants, admitted errors, unnumbered promos relating to the set.
“ what is a master set?”
I define it pretty much with @quuador’s second definition “Every single cards of the set, including variants (e.g. alternative holos from blisters/tins; stamped cards; (Prerelease) promos; Championship Deck reprints; etc. etc.)”, since most people can reasonably attain it, understand it, and it doesnt fall under other term definitions when considered with normal language.
Caution! Horribly pedantic examples: Considered with normal language like being a “full set” can be reasonably understood to be every card in a set, one through whatever, and a “complete set” can be every card in a set that can be pulled from a pack. In normal language someone saying they have a full set of pokemon cards from paldea evolved means they have one of each pokemon, while them saying they have a complete set of paldea evolved cards probably means they have regular versions, reverse, and secrets with “complete” meaning there is nothing else to get.
151 master set collectors… RIP from that Voltorb
Meh, i have a pretty loose definition of master set, depending on the set and how im feeling. My hidden fates set is all cards and reverse holos, plus any main promo cards, but not including every possible variant released. Most of the time i dont go that far. One of every card art in the set works for me. I probably wouldnt do reverse holos for another set, but i try to include major promos. Ig at that point its just a complete set, but master set seems to be an idiom for complete set depending on who you ask i think
There’s probably a line in the sand at some point. I’m not expecting fifty shades of printer hickey, holo shift or extra ink when someone says master set, but it would be truly next level if someone did of course.
I could care less about different languages but I know this is a big thing for many, especially species collectors.
I’m currently completing a Call of Legends master set. I’m including all cards you can pull from packs (inc reverse holos), theme deck variants, pre-release and staff pre-release promos, two triple blister promo’s, luckily only one known misprint (Phanpy misspelling), and all Play! Crosshatch promos. It might be more than others would do with a master set however this will be only master set, as its my favourite, so I wanted to get one that’s in detail as possible.
Great set, hope you can show it when you complete it!
100%! Taking my time with it but hope to be finished by the end of the year or so
I’d also like to humbly submit my own personal offshoot of the master set, what I call the “Art Set”, the concept probably has been raised in other threads before. Basically it’s at least 1 card of every unique artwork in a given set. In practice it winds up being a budget-friendly halfway point between a complete normal set and a master set.
It’s relevant especially for the e-series sets which is where I came up with the idea. In Expedition WotC printed 4 versions of each holo rare artwork: the holo, the reverse foil holo, the rare non-holo normal printing, and the reverse foil of that non-holo normal print. I thought it was pretty stupid and expensive to try and collect so many versions of the same cards, so I designed a visually appealing binder ordering of the set and made a personal goal of collecting every artwork and having roughly half of those cards be either holo or reverse holo. 133 unique cards in Expedition by this definition.
It’s been a really fun goal to chip away at!
This is how I would define it as well. I want there to be a term for “There are no more cards that I could add to this set to make it more complete than it is,” and master is a good fit.
My personal rubric is a “complete set” is where you have collected every card that can be opened from a booster pack. A “master set” implies mastery and cannot be missing any possible pieces. That makes some sets near impossible, but I don’t feel any shame over my Evolutions set missing all the IC stamped cards, or my promo set missing all the Worlds placement cards.