Well, according to YOU, but I'm not in agreement on that point! And this probably marks a philosophical divide that we may not be able to bridge. As long as I've played RPGs, my desire was always to act in a way in keeping with being fully a protagonist (modulus the tension that in a multi-player RPG that role must be shared in some fashion, or I guess potentially some players can be minor characters, though not many RPGs go that route).
This is a good example of when using Wikipedia as a source goes wrong, because Wikipedia blurbs aren't going to give you the appropriate context to accurately discuss a given concept, particularly when the specific Wikipedia page itself is explicitly about conventional writing, and not game writing.
The wiki entry for Video Game Writing brings us closer to where we ought to be, but even still can only give a broad overview, and can't really dive into the specifics were looking for. Though it does make clear the distinct differences that occur due to the interactive and non-linear nature of in-game events leading up to each given plot beat.
After all, that entry only barely touches on the video games that don't feature a fixed plot or a linear level progression (assuming the game even has levels in the first place) at all.
Instead, we would have to seek out more specific sources on the subject, which I can't be bothered to do atm.
Common sense however, tells us that the fundamental differences of the story mediums cannot possibly support the same functions in a protagonist, unless we are deferring to the idea that what is going on and what is desired is just story telling, and as such no characters would count as a game protagonist at all, in turn revealing that the game itself is basically superflous, and we're just doing narrative improv with extra steps and less room for emergent scenes.
I don't recall ever saying it was universally bad, or even bad at all, just not what we're interested in.
This is what we call backpedaling when there's a clear throughline from justifying a certain design to overdefensiveness when pushed back on and on to now claiming the issue was mere disinterest and not a problem that needed to be addressed.
There's no 'phoney jargon' here either
So fun story, I just traced the etymology of this particular Jargon. And I even identified the person who coined it around 2001.
And here's an interesting screenshot from The Forge where this term first got discussed online:
The key highlight here, is the very opening sentence of Greatwolfs reply here. The removal of significant choices.
What other word do we have to describe whats being removed here?
Agency is…
…an actor’s capacity to act in an environment.
…a character’s ability to make meaningful decisions.
…a demonstration of a character’s ability to affect the story.
Character Agency for Beginners
What is character agency? What's it good for, and what are some practical examples? How can I tell if my characters have agency?

As such, if we're to take GreatWolfs interpretation as correct, and we should given Ron Edwards himself said the guy nailed it, and we can correctly identify what he is saying as just being what we already know as Agency, then the problem of Jargon reveals itself.
These forge folks for all their attempts to keep a high level of discourse were reinventing things and making them more convoluted through phoney jargon. It should be telling that while the word protagonism exists in the lexicon, being effectively a descriptive way of saying protagonist, the word "de-protagonism", as originally coined by Paul Czege, isn't recognized by anyone other than people following on from the Forge.
GMs run games, at least in the sort of game we're discussing here, so when you have a GM that is prebuilding a large part of the anticipated narrative structure, in a culture where the convention is strongly for the players to 'play it through', that may or may not be a 'railroad', but it IS in some degree deprotagonizing'.
Not really, because the act of denying agency can only happen when the GM enforces the railroad; it doesn't occur just because the tracks are there.
And anecdotally, theres no shortage of GMs over the years who never had any natural inclination to do it that then come to the forums asking about how to get the party back on track. It isn't the games causing these issues, but the way in which GMs are taught to run them. This is why I question the overall point of pursuing new systems when the system was never actually the problem.
PBTA style games do, for the record, address the actual problem by basically handcuffing the role compared to what other games do, but then add their own, as has been discussed in this topic already.