Join us in the #migration channel on the Drupal Slack team (sign up at https://www.drupal.org/slack) on Friday, July 17. I (@benjifisher) plan to be there at least from 9 AM to 5 PM ET (13:00 to 21:00 UTC).
The community regularly gives migration advice on the Slack channel. This contribution day is aimed at improving the Migrate API and related modules.
Depending on who shows up, we might start work on this issue: Add a migration source plugin for JSON:API or we might plug away at the open core issues for the migration system. There are about 40 that need review (NR).