This feature is used to force some test scenarios to perform application thread eviction, but these usages can be replaced by a fixed "number of worker threads" config.

Also - it is technically "used" by MongoDB, but only to disable scaling behaviour (by configuring both the minimum and maximum to 4). The config can be deprecated (and replaced by a single thread count) without forcing MongoDB changes, but a ticket to update the server's config should be an outcome of removing this.