And IT MATTERS. That's why you have structured concurrency hot on the heels of Loom. And once you are using structured concurrency constructs... Ehh, it's not the magic "solution" we were sold anymore is it?
The naive solution doesn't work because it turns out the difference between functions that return a value and functions that return a promise matters..
The other approaches destroy ergonomics.
template<effect T> T_function List filter(List list, T_function pred)
(element type committed for brevity) and then filter is async if the predicate is async, etc - it's pure if the predicate is, nonblocking if the predicate is. If you continue in this direction, your language gets more complex than C++. The other options are to just accept that you need a separate filter function for async and non async predicates, or to do away with function flags (doesn't work for CPS style async of course).