I am looking at the old code and that allows one to modify it any time.
Giving an error in 10.6 if a slave is running may not be a good idea as it may break things for users (unlikely but possible).
The fact that one could disable semi-sync before for a running slave was a bug as the master is waiting for responses it will may get.

There are other cases where we change defaults in MariaDB that only affects things later.

• Changing global variables only affects new connections
• Changing the default value for binary log format, it will only affect new queries, not old ones.

I do not think we need to change things related to rpl_semi_sync_slave_enabled as we the variable will be default for the next connection.
The current used value can be seen in @@rpl_semi_sync_slave_status.
In multi-master one can have semi-sync enabled for some connection and not for others by changing rpl_semi_sync_slave_enabled when connecting to the master.
In the future we should change things so that semi-sync should be a parameter for CHANGE MASTER so that one can have different semi-sync options for each connection.