Please consider this scenario:

• I am running Moodle 2.9
• I have installed version 1.0 of a non-core plugin which is marked to be compatible with Moodle 2.8 (i.e. not my Moodle 2.9) in the Moodle plugins repo. However, this plugin works fine for me in 2.9, it is only not approved officially (and I can't convince the plugin author to mark it as compatible with 2.9 in the Moodle plugin repo).
• One day, the plugin author publishes version 1.1 of his plugin, but still does not mark it as compatible for Moodle 2.9.
• My Moodle does not notify me about the availability of this plugin version 1.1, although it probably might run as good (and hopefully even better) as version 1.0 on my Moodle 2.9.

I'd like to propose to add a switch to /admin/settings.php?section=updatenotifications called "Notify about legacy versions of non-core plugins" which does:
1. collect a list of plugins which are installed, but are not marked as compatible with the running Moodle version
2. check this list of plugins if there is an update to one of them which is newer as the installed version and marked as compatible with at least the Moodle version the installed version is compatible with
3. inform the Moodle admin about this legacy version within the existing update notification process, making clear that this update is still not marked as compatible with the running Moodle version