I think that Synapse should somehow have the ability to block specific room IDs, as somehow specified by homeserver administrators, from federating (preventing users from joining it or adding aliases or even preventing the room from having aliases).

Use case would be content illegal in the country where the homeserver is ran and potential legal trouble for homeserver administrator for hosting the room or content in it.

I think this issue has been discussed previously at Matrix, but I wasn't able to find an issue either from this repository or matrix-docs and when asking I was told to feel free to file it here.