technically....you need a passport to enter Sarawak because a visa is actually officially needed and granted.

but to facilitate movements of people, a special dispensation is given for stays up to 3 months, to be granted a visa on arrival, and since only an IC is provided, a Social Visit Pass is given as a slip of paper as a surrogate "visa in a passport", which must be shown on exit.

Like I said, the law had not changed, you still need a visa and hence need a valid travel document. If you have a passport, that would probably be the best way.

As special dispensation for movement of people, the use of the IC and issuance of a temporary "travel document" on arrival was introduced about 20 years ago.

All this is because the Borneo states have immigration autonomy and needs to record all entries into the state, both Sabahans/Sarawakians and others. Same as what happens when you arrive from overseas at klia.