EntityReferenceSelection plugin ignored
So, with help from Clive (see his comment below the original question) I have identified two issues:


The correct path for the ActiveProjectsSelection.php file should be src/Plugin/EntityReferenceSelection/ActiveProjectsSelection.php.
The id of the plugin in the annotation should be default:active_projects.
(not related to the original problem) The correct timestamp for the date comparison is DrupalDateTime::createFromTimestamp(time()).


With these changes, my code works perfectly. Also, it does not matter whether I extend NodeSelection or DefaultSelection, it works with either of them.