I wondered this too and found this in the docs[0]:

  Transactions at all isolation levels are subject to lock contention, where a transaction attempts to lock a row that is already locked by a write or
  locking read. In such cases, the later transaction is blocked until the earlier transaction commits or rolls back, thus releasing its lock on the row.
  Lock contention that produces a deadlock between two transactions will result in a transaction abort and a 40001 error
  (ABORT_REASON_ABORTED_RECORD_FOUND or ABORT_REASON_PUSHER_ABORTED) returned to the client.

So looks like you still get a good old 40001 error just like with SERIALIZABLE isolation.

[0] - https://www.cockroachlabs.com/docs/stable/read-committed#rea...