Leiningen has an option to display a fully-populated, well-documented project.clj file (as the sample.project.clj file from its installation.
The Clojure CLI has example-deps.edn which is laid down as ~/.clojure/deps.edn on first use. It does have some examples but it doesn't have all the possible options shown -- in particular it doesn't show where :jvm-opts should live (a lot of people seem to expect it to work as a top-level key in deps.edn despite what the reference docs say about it being part of an alias.
In addition, example-deps.edn still refers to -R and -C and doesn't mention -X or -M.
It also, somewhat confusingly, repeats several aliases from the system deps.edn file as comments, suggesting that those are useful things to uncomment in a user deps.edn file.