Thanks to @Helco's pbw_api_info we can make a list of supported/unsupported APIs in RebbleOS.
Download the SQLite database (thanks to @jwise for making the original database). Unfortunately, the list of supported APIs is incomplete, but will be improved over time.
To figure out which top Pebble APIs are already implemented, you can run:
SELECT apis.name, rebbleapis.name, COUNT(*)
FROM apirefs
JOIN apis ON apirefs.api=apis.id