51% attacks against proof of work blockchains have definitely happened in the past[1]. What hasn't happened yet is an attack on proof of stake (or more generally BFT consensus) as far as I'm aware.
I mean, doesn't Ethereum, probably the most prominent proof-of-stake coin, roll back the consensus whenever something happens that they don't like? It's easy to claim you're algorithm is safe when you're not actually running it.
As far as I'm aware they only did it when they were a proof of work chain