The "touch" command is supposed to pull all data into cache. Only MMAP supported "touch". See docs.

The "inMemory" and "ephemeralForTest" storage engines do support the command, but they just return "OK" as a no-op. All other storage engines, including WiredTiger, return an error, so there's no need to deprecate the command first.

The functionality of "touch" can be implemented by "warming up" the cache with collection and index scans.