Would it make sense to create a trove classifier for projects that follow SPEC 0? And then have a Github robot that files an issue at every SPEC 0 event to remind projects that they are free to drop support for Python/Numpy according to the event?
To make it easy, the issue could have a link to the “what’s new” page for Python/Numpy/etc. and a link to SPEC 0 as a reminder. It could even list the minimum Python versions of the project’s dependencies so that they can see if their dependencies have already dropped old Python/Numpy/etc.
You may want to delay the removal of support of an older Python version until your package fully works on the newly released Python, thus keeping the number of supported minor versions of Python the same for your package.
What most robots do is have a set of commands. In this case, after the robot files an issue, the maintainer could reply @no_more_issues, and the robot would desist. Typically there are other commands like @no_more_issues_one_year, etc.