I’ve got an old site that’s stuck on PHP 7.3. Grav is currently on 2.8.18. Both plugin page and the GF Updates page offer an update to 2.9.9, and will happily auto-update, even though the new version is not compatible. Is it not checking PHP before offering?
This is a WordPress limitation. For updates, the PHP version compatibility is only checked for plugins hosted in the WP plugin directory.