> The bash(1) manpage doesn't state which features are standard and which are not,
I never use the Bash's man page. Bash's info manual has a "Features found only in Bash" section and is much better generally speaking. If, like many other people you dislike the basic info reader there are a number of alternatives like for instance "yelp info:bash"
> and the standard itself is not always easy to find (or read, for that matter).
Every time I have a doubt about standard conformance I just google for "opengroup + sh-keyword" and most of the time I just land automagically on a page that answers my doubt. In the worst cases the answer is only a few further clicks away.