View: https://twitter.com/poken_kibou/status/1379820100253970436
To quote a user on Tumblr,
"Back in the day when PokeSpe ran in multiple magazines, only one of them actually revealed Yellow's identity. Coupled with the fact that the internet wasn't anywhere as big as it is now, it's entirely possible (and likely!) that a good portion of readers thought Yellow was a boy until the volumes came out."
Considering she's shown to have a crush on Red, I wonder if some thought it was LGBT rep at first.