I remember back when one of the fedi users had his nip05 revoked by iris and was banned from the damus relay I'm 99% sure that he was also using the damus client at the time and could still use it. Most clients are FOSS so could be forked if they started doing that like what Tony did with Amethyst when they were censoring accounts by default. It's certainly possible but if you start doing active moderation (the amethyst stuff could be considered passive since it was just based on user end reports without any investigation) potentially in some locales you can become responsible for what you "allow" which is currently borne by the relay operators.