Seems to me they're alive and well. The increasingly powerful censorship forces on places like Reddit has moved at least dozens of communities to create independent platforms for themselves with a variety of codebases. Yes, some of the oldest and worst-maintained ones have died, but many others are still as active as they ever were.
There's also a thing where communities tend to only really migrate to other forum-types that work similarly. Between Reddit/HN style nested threads with voting, classic PHPBB style strictly chronological threads, and Slack/Discord live chat with many channels, good luck getting a significant number of people on one platform type to move to a community on a different platform type. A different software package that provides a matching platform type is a much easier sell.
There's also a thing where communities tend to only really migrate to other forum-types that work similarly. Between Reddit/HN style nested threads with voting, classic PHPBB style strictly chronological threads, and Slack/Discord live chat with many channels, good luck getting a significant number of people on one platform type to move to a community on a different platform type. A different software package that provides a matching platform type is a much easier sell.