Hi @Russell Soper Welcome to Microsoft Q&A thanks for posting your question.
We have the documents available until 15 however all the extensions which are available in 15 should be available for 16 as well.
However, 16 is not listed version yet on official Microsoft Doc
you can also refer the similar ask here https://video2.skills-academy.com/en-us/answers/questions/1432093/pg-buffercache-extension-not-available-in-postgres
Regards
Geetha