REVOKE IF EXISTS makes a lot of sense. We had MDEV-5359 that was about supporting IF EXISTS everywhere, but we forgot to cover GRANT and REVOKE.

On the other hand, IGNORE UNKNOWN USER looks strange, not consistent with existing syntax. I'd prefer to use IF EXISTS too. Like

REVOKE IF EXISTS SELECT ON *.* FROM IF EXISTS foo@bar

This isn't very English, FROM foo@bar IF EXISTS would look better.
Compare with

ALTER TABLE IF EXISTS t1 DROP COLUMN IF EXISTS col1

I don't think the FROM IF EXISTS (and IGNORE UNKNOWN USER) has much usefulness on a REVOKE IF EXISTS statement as the privilege(s) could not exist (and thus, be removed) if the user doesn't, it could simply throw the same warning if the privilege(s) and/or user doesn't exist.