It hasn’t been profitable to mine with GPUs for years now…
Depends entirely on what you paid for the GPU's and the energy.
A 4090 can do tens of GH/s on SHA256D (BTC) for a substantial capital investment and 300W - other GPUs substantially less.
A $10 ASIC can do >500 GH/s for 5W.
If you possess these GPUs, at any energy price it becomes orders of magnitude more cost-effective to mine something else, rent for LLM uses, etc. and then just buy Bitcoin.
I thought ASICS only worked on certain protocols like BTC, whereas ETH was implemented to intentionally limit their utility and require more general purpose hardware. Is that false?
BTC ASICs only work on BTC. ETH no longer operates using proof-of-work, all of this equipment moved to other coins (many of them using the same algorithm.)