A while ago, in topic #18244, the tag fictional persona was brought up by thelieutenant, who described it as "meant for OC characters in video games, but it is extremely underutilized. All OC trainers, player-created MMO characters, etc should have it, but it would require an incredibly large amount of gardening."
This would prompt the remainder of the topic to deal on elaborating further on the tag, with 7HS commenting on the way the wiki had been described the tag by that point, later on updating the wiki to fit more the description thelieutenant had given it. While I think this is a positive change, likely to aid in the tag's continued use, 7HS noted something important about the tag as it was: "Well, the former [example] might also get tagged artist_self-insert. I think the current fictional_persona is just a subset."
Because that's what fictional persona was (and for some, is), a sub-set of artist self-insert.
The original wiki described the tag as "When an artist develops a fictional character that is meant to represent oneself," with it naming examples like one's MMORPG character or a version of themselves made via a character generator (ala shindan maker). It also explicitly "goes in hand with self-portrait". In effect, while artist self-insert is for all instances of an artist self-inserting themselves, fictional persona was for any self-inserting designs which weren't just a caricature of their real-life self (that's also why the only Other Name the tag has is ファーソナ, fursona). That's why the tag was extremely underutilized, because for most folks it is just redundant when many artists already have avatars/self-inserts that have nothing to do with their own appearances. As a result, when users tagging commissioned art of MMORPG player characters noticed that fictional persona seemed applicable, the definition for them began to shift when it didn't for others.
One can see its biggest failure in use when looking at perhaps one of the most noteworthy examples of artist self-insert on Danbooru, Mizuki Hitoshi's Hang in There, Kogasa-san. When looking at the wiki for that tag (and real life insert), it mentions that an artist self-insert "can also have an existing character acting as a fictional persona of the artist," but despite the fact that Kogasa-san holds just about half the posts accounting for artist self-insert, only one of those posts has fictional persona tagged (likely due to the use of Momiji to represent Yume Shokunin). As a lesser example of failure, a number of wikis linked to fictional persona when referring to similar cases, but in nearly all instances, the tag was either nowhere to be found or tagged a negligent amount when compared to artist self-insert (so many wikis in question have had the tag dummied out to avoid stirring any further confusion here).
Of course, now begs the question - what to do with the tag? Clearly, the new definition the tag has received offers it far greater utility than its previous definition, especially for those interested in finding player-made MMORPG characters, but the previous definition still lingers elsewhere on the site, and within the tag itself. The tag itself was originally intended for an artist's fictional persona, not all fictional personas (commissions of which could theoretically go under borrowed character), and wasn't restricted to settings not of their own creation, so for all intents and purposes, a tag gardening effort has to be organized to clean it up. Additionally, while a tag under its original definition may be redundant to some degree, we don't have general tags that collect together things such as fursonas and the like that would go under a tag with that definition, and you can't put it under artist self-insert because the art could also be commissioned, thus theoretically belonging to borrowed character despite that being for artists drawing another artist's character. Something like alternate persona or something...
Updated