If you have no preferred pronouns, just say (none) or something
This will help make people feel more comfortable in this comm and has a side effect of showing you support trans people when making comments in other communities. It will also make reactionaries seethe and make it easier for mods to purge them, and make trans people feel safer all over the fediverse.
I know I have it in my bio, because I am in agreement with it since before you made it a rule.
Your own headline says that not all instances have this as an option. That makes it unenforceable for those instances. It is also unenforceable for anyone wandering in from a main page, because they won’t be reading this rule before posting. And then sure you can delete their post I guess but what if it wasn’t transphobic?
And if you don’t know Bill Maher, good. Just know that he is a ghoul and loves to say ‘new rule’ and then spout the worst take you have heard in your life on a weekly basis.