Currently, there is no way to force function fdef validation on, outside of tests and immune to override with dynamic vars. Forcing a validation in a function can be done using either with a) re hook or b) a manual s/conform / s/valid? call but those do not contribute to the function documentation. There are many use-cases for functions that always validate the inputs, e.g. when configuring components on application startup, done once where the performance penalty doesn't matter and correctness is important.
Schema has the :always-validate metadata for this case:
Maybe something like: