Ask Different is a question and answer site for power users of Apple hardware and software. It only takes a minute to sign up.
Sign up to join this community
After a forced reboot (and another reboot for good measure), the applications I allow to "Accessibility" control under System Preferences > Security & Privacy > Privacy no longer includes Rectangle.app nor allows me to add it manually.
Nothing appears there anymore and nothing can be added. 
Fortunately I've solve my issue. The solution is frustrating.
What reboots could not fix, a shutdown and start did. Know that system behavior after a reboot may differ from system behavior after a shutdown and start up, manually sequenced.
In evidence, here are some of the Accessibility authorized apps after restart, including the Rectangle app of concern.
