Who writes the recommendation letters for returning student PhD applicants after a big hiatus? The answers to previous questions indicate it's possible to apply for and complete a PhD even at age 65+ (example, another example).

If these applicants did undergraduate studies at a conventional age of 20+, by age 65+, their undergraduate supervisors are probably retired or deceased. Who writes the recommendation letters then (especially if the applicant's job did not involve research)?

If the answer to this question is also applicable to all students whose supervisors are retired or deceased (e.g. a campus shooting kills all of a student's supervisors), I'll edit the question to include the broader scope.
I am not aware of a specific rule requiring academic references. It may be more a matter of common sense. If an applicant has recently completed a lower degree, the committee may expect to hear from professors who saw the applicant's work.

I was admitted to a PhD program over 25 years after my master's degree was awarded. What I had done in the last quarter of a century was far more relevant than my last academic experience.

I got references from my manager, my prior manager, and a chief technology officer. The last person was less familiar with the details of my work, but had a PhD and connections to the department where I was applying.