We may not know how certain initiatives are important. This is used for example inhttps://pypi.org/project/tldextract/, which also accidentally is used by my project.
Weird selection. Why does it not include say .gov.uk and .ac.uk even though it includes obscure ones like .api.gov.uk? Clearly they aren't limiting it to just domains where the public can register.
In addition to the fact that .gov.uk and .ac.uk does exist in this list, which has two logical sections where the first section is for official TLDs and the second section is for custom domains, the PSL is effectively being used for additional origin protection: foo.gov.uk and bar.gov.uk should have separate origins because, of course, they can and normally be operated by different entities. So as long as such things can happen, the public registration is not really mandatory for being included to the PSL. (One particular exception is that a generated domain for all possible IP addresses [1], which technically benefit from the inclusion but the maintainers don't want to endorse such unsafe services.)
As another side note I capture not only tlds, but also domains in https://github.com/rumca-js/Internet-Places-Database