myisam_recover is said to be a valid name, but it is not

https://mariadb.com/kb/en/mariadb/myisam-system-variables/#myisam_recover_options

myisam_recover_options
Description: MyISAM recovery mode. Multiple options can be selected, comma-delimited. Using no argument is equivalent to specifying DEFAULT, while specifying "" is equivalent to OFF. If enabled each time the server opens a MyISAM table, it checks whether it has been marked as crashed, or wasn't closed properly. If so, mysqld will run a check and then attempt to repair the table, writing to the error log beforehand. Previously called myisam_recover, which is still a valid synonym.

There is no such synonym (at least not anymore, I don't know for which version it was written). This form works as an option not because it's a valid synonym, but just because it's a prefix; and since nowadays we rather discourage people to use prefixes, there is also a warning about it in the log.