What's the low-down on MariaDB JSON support?
No plans that I know about.

Regarding constraints, I assume you mean for JSON validation: See Changes and Improvements in MariaDB 10.4:  


  The JSON_VALID function is automatically used as a CHECK constraint
  for the JSON data type alias in order to ensure that a valid json
  document is inserted (MDEV-13916)


... so you don't have to set the constraint explicitly.