On the Internet, that storehouse of dubious advice, you can still find statements about grammar and usage like this one: "One is still officially supposed to avoid ending sentences with prepositions. In most cases, this is not hard to do: 'Who are you going with?' becomes 'With whom are you going?' Or, 'I was making cake and decided to put chocolate chips in' becomes 'I was making cake and decided to add chocolate chips.' "
Let us be clear. The advice against terminal, or stranded, prepositions comes for eighteenth-century attempts to make English "correct" by making it resemble Latin. But the terminal preposition has been normal in English since Chaucer was a schoolboy, as evidenced in the classic example: "What did you bring that book I didn't want to be read to out of up for?" It's not elegant, but it is not word salad either. It is comprehensible because it is idiomatic English.*
No reputable authority insists on enforcing this superstiion of usage. Even Wikipedia (!) indicates that it is a superstition rather than a genuine rule.
So I issue a challenge. Find me someone who would change the sentence They don't know what he has been up to to They don't know up to what he has been, and if I can't demonstrate that you are being advised by a braying jackass, I will eat my head, sir.
The superfluous terminal preposition is still a mark of subliteracy, as in the well-worn joke about the young man at Harvard who asks, "Where's the library at?" and is told not to end the sentence with a preposition. Surely you already know the punchline.