What can I do to find out since which release a given DBMS supports a feature, function, etc?
Normally I would just google with some specific keywords. If you know the official website of the given DBMS, that could be even more helpful.

Example: To find out which version or release of Oracle the CTX_DOC.SNIPPET function comes from, just google: oracle new CTX_DOC SNIPPET function site:oracle.com .

Sometimes the last resort is probably the easiest way. You just need to fine-tune your search terms.