Quoting https://source.wiredtiger.com/develop/arch-timestamp.html:

Applications are expected to write the value sequence in order; that is, for each key in the database, every write should commit with a timestamp later than the latest existing timestamp for that key. This is currently not enforced; because MongoDB sometimes issues out of order updates, they must for the time being be handled.

The last sentence "This is currently not enforced; because MongoDB sometimes issues out of order updates, they must for the time being be handled" is no longer valid and can be removed. We should probably also mention or refer to the write timestamp usage enforcement options and its default, which are described in https://source.wiredtiger.com/develop/struct_w_t___s_e_s_s_i_o_n.html#a358ca4141d59c345f401c58501276bbb under "write_timestamp_usage".