Even if this rumor is true, it doesn’t even mean the entire movie will be a 90s period piece, as it could simply mean that part of the movie could flashback to the decade and show a young Black Widow, approximately 16 -years-old based on what we know of the character, perhaps with a storyline that connects to the modern day.
Still, with the upcoming Captain Marvel movie being set in the 1990s as well, the idea that a Black Widow movie could be set at the end of the same decade is certainly plausible.
The rumor comes via MCU Cosmic from an unnamed source, and so grains of salt must be taken because we can’t even begin to verify it.