That just encourages people to keep using old, unmaintained, insecure versions of libraries. Then, when they're still on version 2.1.1, and your maintained version is 5.7.3, and somebody finds a major security bug in 2.1, they will come whining at you to release a 2.1.2.
Code that is not being maintained is not usually suitable for use, period.
And then you can offer them a support contract to produce an update for an out of support version
Library maintainers have no right to police how people use their open source code, period. Maintainers are also not obligated to backport security fixes. Anything else is effectively against the concept of open source.
Notably, even this policing doesn’t fix the whining. The whining will just be about what TFA is whining about. You’re just moving the whining around.
It also does nothing to actually force people to upgrade. Instead, people can just cap against the minor version you broke your package on. Instead of being user hostile, why not make the user’s job easier?
Correctly following SemVer disincentivizes unnecessary breaking changes. That’s a very good thing for users and ultimately the health of the package. If you don’t want to backport security fixes, users are free to pay, do it themselves, or stop using the library.