How to drop an in-memory temporal columnstore
It seems you have to drop the columnstore index first.  This works for me:

ALTER TABLE A DROP INDEX IX_A_CCS;
ALTER TABLE A SET (SYSTEM_VERSIONING = OFF);
DROP TABLE A;


Note that this leaves behind the temporal history table, so you might want to run DROP TABLE A_History; for good measure (which is the default naming when a history table name is not specified).