It is in the documentation now (at least as of oracle 19).

drop_constraint_clause::=

drop ( 
  primary key |
  constraint *constraint_name* |
  unique (*column*, ...) 
) ... [ (drop | keep) index ]

(I did not want to copy paste the image, as I am not sure that's okay with oracle; I hope the notation is self-explanatory).

https://docs.oracle.com/en/database/oracle/oracle-database/19/sqlrf/ALTER-TABLE.html#GUID-552E7373-BF93-477D-9DA3-B2C9386F2877__I2103997