There is a useful function in PostgreSQL 9.5 called aclexplode() but I can't seem to find any official documentation on it.
Should I avoid using it? Is it technically an unstable API?
Yes, you can rely on it. This function is finally documented in upcoming PostgreSQL 12 (commit).
aclexplodereturns anaclitemarray as a set rows. Output columns are grantoroid, granteeoid(0forPUBLIC), granted privilege astext(SELECT, ...) and whether the prilivege is grantable asboolean.makeaclitemperforms the inverse operation.