can changing fast flags get your roblox account banned?

Editing them yourself isn't really a ban thing anymore. Roblox moved to an allowlist last year, so the client just ignores any flag that isn't on the approved list - the edit does nothing rather than getting you flagged. The risk is the tool, not the flag. Anything that injects into the client gets treated like an exploit. Sticking to the allowed flags through a well-known launcher has been fine for everyone I know.