I find all of this revisionist history interesting. Guess that is what happens when I disappear for a few years.
Shadowless was known and named while Unlimited was being released. I was collecting Pokemon as my kids played (at the local Zany Brainy) and noticed it as a variation. I got into a battle with A/B/U in 1999 over a Charizard holo I needed to complete my first Shadowless set. This was early days of Ebay and they offered the holo I needed. Their price was expensive for an Unlimited Charizard but I was willing to pay the premium to complete the set, since I had no idea how rare it might be. Based on the artwork I placed an order (after calling them to confirm the artwork was of the specific card). Turned out it was stock art showing a Shadowless card and they sent me an unlimited card instead. I demanded a refund and as part of my explanation of why I described the card I wanted as “shadowless” and the one the sent me as “shadowed”. I had to send them a picture and walk them through the difference. So the variation was known to some of us as soon as the cards came out. I spent the next year buying full sets based on how many shadowless cards they had.
There was a long-time Pokemon seller on the West Coast (I think his name was Gene) who I discussed “shadowless” cards with at the time. He specialized in selling WOTC sets on Ebay, and did so for many years. He sold sets of all condition, and his experience (like mine) was that Shadowless was much rarer than 1st edition, perhaps as low as 50% or even less. Over the years Gene sold over 200 1st Edition base sets but only about 100 shadowless sets.
The story we heard in the early 2000’s was that all the original run (1st edition and Shadowless) were supposed to be 1st edition. The 1st Edition stamp was a separate, independent process, after the card was printed. WOTC didn’t have enough packing materials to package all the 1st edition cards. Originally, they were going to get more boxes and booster foil. But Pokemon exploded when it was released in the US. Rather than hold up distribution by holding back first edition cards, WOTC decided instead to not stamp the remaining 1st Edition sheets. The last part of the print run (or maybe a smaller second print run) was packaged as Unlimited and shipped out to meet the demand. That bought WOTC the time to adjust the artwork and text as necessary for the much larger Unlimited run, which was huge by comparison to both Shadowless and 1st edition.