I did some deeeeeep diving, and not that everyone wants to fall down the rabbit hole of those links LOL, but the reason people think it's a prequel is there was a fake spoiler with a synopsis that actually goes back to the script David Hayter wrote in 2004. There's a good roundup of that script here https://www.cbr.com/movie-legends-revealed-black-widow-nearly-beat-iron-man-to-the-big-screen/
If a script was written in 2004, that makes it a prequel about Nat's coming in from the cold a lot more likely. But the truth is AFAIK there have been no real spoilers about the BW movie (some supposed set photos on Twitter recently were from another project). Nicole Perlman wrote a treatment (not a script) in 2010, but said on Twitter in 2014 that she wasn't developing it. There wasn't anything really definitive about any BW movie until the news broke in Hollywood Reporter in October 2018 that ScarJo got paid $15M for a single standalone movie that she's producing. They went through something like seventy directors in six months and were thinking about hiring a man, until she brought in Cate Shortland in December 2018. (Lucrecia Martel said Marvel talked to her about directing the film, but said "They also told me, 'Don't worry about the action scenes, we will take care of that.'") Jac Schaeffer, who wrote The Hustle, co-wrote Captain Marvel movie and who's going to be the showrunner and pilot writer for the WandaVision series, had written a new script. Now Ned Benson is rewriting it, which is probably why the shooting was pushed back to June of this year. Scuttlebutt is Benson is adapting a story about a woman bringing down the Aryan Brotherhood in Texas and ScarJo was attached to it, but who knows. The Russos are now retired (they say) and have nothing to do with it, and the writing team said they weren't even sure when drafting EG (i.e. Nat's death) if there would be a BW movie. (They said something hilarious like "Well, they knew what we wanted to do and didn't say anything about not doing it.")
tl;dr There are a lot of Black Widow ideas floating around (Nathan Edmondson wrote the opening pages of a script and posted them for free on his website a while back) and the prequel spoiler is really dubious.
Depressingly, a whole lot of stories about Nat are only about her Red Room backstory, so that'll probably factor in anyway.
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If a script was written in 2004, that makes it a prequel about Nat's coming in from the cold a lot more likely. But the truth is AFAIK there have been no real spoilers about the BW movie (some supposed set photos on Twitter recently were from another project). Nicole Perlman wrote a treatment (not a script) in 2010, but said on Twitter in 2014 that she wasn't developing it. There wasn't anything really definitive about any BW movie until the news broke in Hollywood Reporter in October 2018 that ScarJo got paid $15M for a single standalone movie that she's producing. They went through something like seventy directors in six months and were thinking about hiring a man, until she brought in Cate Shortland in December 2018. (Lucrecia Martel said Marvel talked to her about directing the film, but said "They also told me, 'Don't worry about the action scenes, we will take care of that.'") Jac Schaeffer, who wrote The Hustle, co-wrote Captain Marvel movie and who's going to be the showrunner and pilot writer for the WandaVision series, had written a new script. Now Ned Benson is rewriting it, which is probably why the shooting was pushed back to June of this year. Scuttlebutt is Benson is adapting a story about a woman bringing down the Aryan Brotherhood in Texas and ScarJo was attached to it, but who knows. The Russos are now retired (they say) and have nothing to do with it, and the writing team said they weren't even sure when drafting EG (i.e. Nat's death) if there would be a BW movie. (They said something hilarious like "Well, they knew what we wanted to do and didn't say anything about not doing it.")
tl;dr There are a lot of Black Widow ideas floating around (Nathan Edmondson wrote the opening pages of a script and posted them for free on his website a while back) and the prequel spoiler is really dubious.
Depressingly, a whole lot of stories about Nat are only about her Red Room backstory, so that'll probably factor in anyway.