You could use $wpdb->query() to run a delete query of all records with your particular meta_key. I don’t know if this is the “right” way. It gets away from the need to loop through user IDs, but it can be very dangerous, there is no undo query. 🙂 If there is some accidental name collision, you could wreak havoc.
There is no difference in usermeta tables between regular and MU versions, so there are no additional considerations I can see.
You probably know this, but it’s worth noting, this should only be done on an uninstall hook, NOT the deactivation hook, and that it’s very clear to the user that uninstalling will forever delete these settings with no chance of recovery except by restoring the DB from an earlier backup.
Yes, and uninstall is always system-wide. Then I can also delete a query and not all have to go in a loop over all users. So I’ll do it now 🙂
Thank you
greeting
frank