These checks were added in [51193] and [51197] as a temporary measure to prevent a site owner from activating a Full Site Editing theme when Gutenberg plugin was not active.
Because Core only had partial support for the features required for FSE, the front end of the site would display a white screen or error message to visitors. Unless the site owner visited the front end, there would be no indication that there was a problem.
Since 5.9 will include the remaining features required for FSE, these checks can be removed.
Props poena, jffng.
Fixes#54366. See #534190.
Built from https://develop.svn.wordpress.org/trunk@52044
git-svn-id: http://core.svn.wordpress.org/trunk@51636 1a063a9b-81f0-0310-95a4-ce76da25c4cd