On first glance, this seems more surprising than useful to me.

CI: https://ci.nodejs.org/job/node-test-pull-request/45669/

Some additional elaboration on the purpose here: this PR is partially a continuation of #43196 and attempt to work around its inability work with functions.
Functions are mutable, and we almost never want them to be mutated. An accidental mutation is possible, for example, if we pass a callback to "overloaded" method with complicated logic to determine which "signature" is needed. It can potentially happen only in case of error, without calling the callback - making it harder to catch.
Fortunately, most of the time tests are using common.mustNotCall() or functions wrapped into common.mustCall() or common.mustSucceed(). Freezing returned functions allows us to:

• Ensure that test code doesn't mutate functions by wrapping them
• Have this implicit check across a lot of places in already existing tests

This is very implicit so I'm not 100% sure about this idea; feel free to block if it sounds dangerous or completely useless.

On first glance, this seems more surprising than useful to me.

I agree, the name doesn't give any indication that this might happen, and it feels weird that there is no way of disabling it.

Not really seeing the usefulness here.

Closing as unneeded. Adjustments to test-timers-user-call.js might make the test more strict but it won't improve what is tested. Freezing functions might be done independently to perform overall more strict runs, but it doesn't fit main branch. Thanks for reviews!