Current patch causes trouble in case the client inadvertently has such a tileset active that it triggers initial server setup with wrong ruleset.
The new version brings the restriction only to is_restricted() connections - so the client that forked the server can still change the ruleset.
If freeciv-server has been launched with specific ruleset given at commandline, it makes sense not to allow (regular) users within the server to change that.
As current behavior is not strictly a bug, and some users may rely on it, targeting this to master only, as a behavior change.