First reported by IGN, it appears that Marvel Rivals' season one update has disabled mods in the game. Until now, there was actually a surprisingly lively scene of modders creating skins and other cosmetic additions for the NetEase hero shooter.
Users in the Marvel Rivals Discord have said that the update has introduced asset hash checking to prevent client-side modification of the game's files.
There was no mention of this change in the season one patch notes, but that's probably because NetEase never saw modding as a legitimate player behavior: Modding is verboten according to the game's terms of use, and NetEase told IGN that players who mod their Marvel Rivals installations risk being banned.
It's kind of a bummer to see, but this felt inevitable—it was far more weird to see a free-to-play, cosmetics-driven, live service multiplayer game allow cosmetic mods in the first place.
Client-side skin mods are largely harmless, but competitive games are understandably skittish about any backdoor to creating an unfair advantage.