It appears, on the whole, to be an archaic usage, but here’s an OED citation from 2007:
The Riemann tensor tells us all what we need to know about the
intrinsic curvature and thus gravity.
Practical advice might well be to use all that, but if you want to use all what, then there are many precedents.
The OED’s definition 7 of what is ‘As simple relative (sing. or pl.): Which (or who); that.’ Within that, the first sub-category reads:
a. referring to a pronoun (demonstrative or indefinite), occasionally
to a noun; originally introducing an indirect question in apposition
to it especially, in later use only, in all what (now dialect or
There are 14 supporting citations under this entry. There are others elsewhere in the OED illustrating all what, including one from ‘Timon of Athens’.