I don't have any decent proposal at the moment but I think coming up with a way to annotate side-effects of functions (including typeguard-ness) could come in handy. If we anticipate needing that, perhaps it would be beneficial to come up with that feature before implementing this PEP, lest we end up with something that could have benefitted from it but was released just before it.

Though personally I like the PEP and have no qualms about having to learn that TypeGuard is "a bool with a side-effect"; I don't think it's a problem in the first place, there are less obvious, more complicated things in Python that I couldn't just intuit at a glance.

[...]
but this also isn’t quite accurate, and we were concerned that this might be highly inconvenient in practice.  In a sense, the type guard-ness of the function is an attribute about the function, not about the parameters or return type, but there is no way to currently express that using Python or type checking syntax.

[...]
Cheers,
-Barry (on behalf of the Python Steering Council)