You mentioned WotC era cards, but FYI: all six of those LPPCollecting stamped ivy Pikachu were printed after the WotC era had already ended. The regular Giogatori, Giocatori, and English ‘Welcome to the first run’ stamped versions were stamped and distributed for the Italian Pokémon Day on June 18th, and 19th, 2011. The upside-down Giogatori and staff-exclusive upside-down Giocatori were printed and distributed somewhere in 2017. And the 20th Anniversario stamped versions were given from December 2nd, 2021 onward by spending at least 50 euros on their webshop. (And they also have some 21st and 22nd anniversary stamped cards, but no ivy Pikachu, only more modern cards.)
Here all the Pikachu of my personal collection (missing the staff one):
And here a picture of some other LPPCollecting stamped Pokémon cards I saved pictures of in no particular order (they also have stamped MTG and Yu-Gi-Oh cards):
As for some other unofficial/unlicensed stamps I’m aware of:
1) IL Ritrovo Bresso stamp English EX FireRed & LeafGreen Pikachu. These were stamped by an Italian shop called IL Ritrovo in the town Bresso, which were also distributed at LPPCollecting Pokémon Day events. Here a picture of the Pikachu (I own all three EX era codes for it, but have yet to make a picture of them):
2) Y2K stamped Base Set Pikachu and Energy cards. To celebrate the New Millennium, 2000 English unlimited edition Base Set Pikachu cards were stamped with “Y2K” alongside a random Base Set Energy card and Certificate of Authenticity. No idea where they were given, but since they celebrate the new millennium, I think it’s save to say they were given somewhere in January 2000.
Here the Pikachu and Psychic Energy from my personal collection:
3) The DP era distributor stamped cards of San Diego 2007, New York 2008, and Chicago 2009, all three given to attendees at the Pokémon USA distributor meetings, with less than ten copies made. Here an image I saved of all three, plus an additional Chicago 2007 stamped Chimchar that emerged a few months ago. Coincidentally, an active thread was posted yesterday with some more info about these.
4) The COLLECTOR stamped Dragon Majesty Charizard, of which roughly 400 copies were stamped and given to attendees of the Collectors Event in Hartford, Connecticut between May 3rd and 5th, 2019 (although this took place at the same place and time as the Pokémon Hartford Regionals, both events were not related). Here a picture of the Charizard I own:
5) French Vermot 2018 Star-stickered and PCA-authenticated Ash’s Pikachu promos. On December 8th, 2018 in Paris, at the Vermot et Associes auction house, there was an event where they’ve auctioned off some PCA-graded cards. PCA (Professional Cards Authenticator) is the reasonably well-known French grading company that was founded in 2016. During this auction, a total of 54 PCA-graded cards were auctioned off, and each auction winner would receive an additional French SM109 Ash’s Pikachu promo. These Pikachu promos had a large golden five-pointed star sticker, which were then authenticated by PCA, labeled as “LOT 10##”, where the “##” is a distinct number. I’m not sure if it goes from 1000 to 1053 or 1001 to 1054, but I personally own 1009 myself and have seen 1053 before.
Roughly 10-15 copies (unfortunately I don’t know the exact amount) of the SM114 Ash’s Pikachu promo with the same golden star exist, labeled as STAFF. These were given to the staff members who helped organize and manage the event.
And lastly, 10 copies of the SM112 exist, labeled as GIFT. These were given to the people who filmed and photographed the auction.
Even though there are seven SM Ash’s Pikachu promos, only these three were used for the event promos.
Here all three of the Pikachu from my personal collection:
6) Christmas '99 - Pack 729 stamped unlimited edition Base Set Pikachu and unlimited edition Jungle Jigglypuff. Cup Scout Pack 729 is a youth scouting (Boy Scouts of America) group in Westland, Michigan. These were likely given as a holiday gift at the time, although no idea how many were distributed. Unfortunately, I don’t have this Pikachu in my collection yet, but here are two pictures I’ve saved of both:
7) Hungarian MINTA stamped English unlimited edition Base Set bootleg cards. Figured I’d mentioned them, even though they’re not real cards with unofficial/unlicensed stamps, but custom printed cards with MINTA-watermark instead. The ones I’ve seen thus far are a Wartortle, Magnemite, and Gust of Wind, all from this thread earlier this year (which also contains some info in the comments about them). Here the saved pictures from that thread:
8) “PM” stamped English unlimited edition Base Set Pikachu and Japanese Leaders’ Stadium Misty’s Tears. I honestly don’t know anything about these (I’m not even sure if the PM stands for Pocket Monsters, Pokémon Master, or something else 
), and I’m also still missing this Pikachu unfortunately.
I think those are all the unofficial/unlicensed stamped cards I’m aware of. 
Greetz,
Quuador
