Do we allow functions for any rcParams?

Not sure. If so, only when setting it at runtime. Obviously, one cannot set a function in config files. But if there is no technical limitation I don’t See a reason why who shouldn’t allow that at run time.

As long as the function in importable, we can do something like 'function://module.module:function' as we do with backends.

I suspect our logic when deprecated this was that if you can not express it in the config file, it should not go in rcparams

I guess that the example in the documentation actually tells you how it should be done if you want a function. If I recall correctly, I was just trying to test out some things for code coverage and maybe didn't really read the documentation that much in detail, but thought that it should be possible as I probably copied some other test code...

So if the conclusion is to close this as it is OK and the documentation says so, I will not argue. (Possibly one can add a note that the only way to get a function is to do as in the example, now it can, maybe, be interpreted as one way to use a function, not the way).

As long as the function in importable, we can do something like 'function://module.module:function' as we do with backends.

I wouldn't do that unless we have very good reasons. This is a proprietary pattern users would have to learn (reduces the chance that it gets used) and we would have to maintain it.

Let's not take any actions here until users are asking for it.

@oscargus feel free to clarify the documentation.

Ah, sorry. I'm against parsing functions from Matplotlibrc. It would be ok to modify the validator to support runtime-assignment.