The process of merging d-cakeday into core didn’t go as planned…
All bundled plugins have to be disabled by default, and d-cakeday was one of the few migrated that have always been enabled.
The idea was that sites that had the plugin enabled before the migration to core would still have it enabled, while those that didn’t have the plugin before would use the new default. (off)
I’ve now opened a PR that replaces the most recent (incorrect) migration with a new one that uses a heuristic to determine whether to enable cakeday/cakeday_birthday or to keep the default “off” value.