Seems that too many translations was removed in https://github.com/umbraco/Umbraco-CMS/pull/16050, this is because the keys for healthchecks are not directly referred, and thus does not come up as referenced.
How to test
Follow steps in original issue, there should no longer be translation keys "[HeaderNotFound]" but proper text instead 👍
Fixes https://github.com/umbraco/Umbraco-CMS/issues/16479
Notes
How to test