As always, there's a good group of folks on reddit.com/r/sml and StackOverflow is surprisingly active as well (maybe due to the number of students still using Standard ML). There may be some good IRC channels but I'm not as familiar with them.

For a recap of the current state of Standard ML (also surprisingly active), here's a roundup from last year: https://notes.eatonphil.com/standard-ml-in-2020.html.