There's some pretty in depth breakdowns further back, but I understand it's annoying to dig through old posts. Ayumi could probably give the best summary, but I can give it a shot.
IIRC, back around April 2019, a guy on a Japanese forum/board claimed AoCF would be ported to PS4 and Switch, and that Reimu would be included in Smash. Now this wasn't the first rumor claiming Reimu, but there was reason to believe this person was privy to inside info. For one, shortly after this post Tasfro confirmed AoCF was coming to PS4 and that they were considering porting the game to Switch. Additionally, there's evidence to suggest this poster also leaked pages from WaHH early (though my memory is more hazy on that count).
After this, there was a long period of silence. At some point during late summer/early fall there was another post suggesting Reimu would come with Yukari as an Arsene-like assist plus a Sanae+Kanako echo, but this was just the poster's own "fantasy;" this guy has a habit of mixing speculation with rumor. I'm pretty sure this was the same post that put the "doujin" in doujin leak/rumor (though it may have been the original) because it was stated that music from doujin circles would come with Reimu and that her inclusion was something of an "open secret" among circles.
Then is late September-ish an interview with ZUN was released where he said unprompted that he would like to see Reimu in smash, although he doesn't think it was likely. What's interesting is that the interview was recorded back in April, which as best we can tell, is before FP2 was finalized. But it's also around when the first post was made, so if the poster really is in the know for Touhou then they may have had early access to that interview.
Since then, there have been minor posts likely from the same person assuring people that Reimu's on the way, but nothing substantial. The "delayed by Covid" was the last post and that was several months ago.
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Here are my thoughts on the doujin rumor: as it stands, taking the 1st post at face value it's probably fake. If we were supposed to take it that Reimu was in development at the time it was initially posted, then too much time has passed. I'm sorry but Covid delay or not, I don't believe it would take 18 months to develop Reimu, no matter how complex the concept. In that time span she should have been in FP1.
Frankly, it's pretty easy to make a fake leak with text because there really isn't any way to corroborate it, especially if the rumor itself is pretty vague. On top of that, all leaks and rumors have a shelf life, and after more than a year chances are it's gone sour. I've said for a while the doujin rumor gets more unlikely with every passing day, and at this point I think the matter's settled.
One explanation is that the poster was lying intentionally to troll or drum up speculation (this is known to have happened in unrelated instances, looking at you JSON "leak"), and subsequent post ("fantasy" aside) are attempts to save face or keep the lie going. While probable, this reason is boring and kinda contradictory considering the follow-ups. I think it's possible this person earnestly believed Reimu was on the way. To that end, I've had two ideas that aren't mutually exclusive: it's possible that he did have early access to the ZUN interview but misconstrued ZUN saying he wanted Reimu in smash to men she was in smash; alternatively, Reimu being in smash may have been an inside joke among doujn circles (e.g. "Do you think Reimu's coming to smash?" "Oh definitely, I just got done recording the song that's gonna be in her trailer LOL") that the poster heard out of context and assumed was serious.
Now, what does this mean? Really, all this means is that one rumor saying that Reimu is in Smash is most likely bogus. Ultimately, it has no bearing on whether or not she actually makes it in. The only impact it can have is Reimu's perception among speculators, that's it. I'd say in that regard it's been a net good, because aside from people angry about bandwagoners (which, trust me, I understand) her visibility has shot up dramatically, among both Smash fans and Touhou fans.