Were anyone-can-spend transactions valid before SegWit?
there was nothing to &quot;prohibit&quot; a miner from spending/stealing someone else's funds

That was never true, so far as I know, miners have never been able to spend other people's funds seen in normal transactions with the right spending scripts.
A more accurate description of the situation you describe is perhaps that there was nothing to prohibit people from freely leaving their money unprotected in a public place for any passerby to take.