FPGA for development, AWS can afford to order custom ICs.

Could surplus crypto miner GPUs be repurposed for this?

No

With enough gates you can do anything on an FPGA

How much would someone pay for this? I have a half-written zstd core, but I doubt the market for $100-150 FPGA-based compression accelerators is all that large.

Probably not worth it as a FPGA solution or even in general as an add-on card (the overhead of dealing with such extra hardware means that the threshold for "worth it" is very high).

I would expect this to become part of newer generations of CPUs once it becomes popular.

Intel sells a gzip PCIe card called QAT. Not many seem to be sold.

I'd never heard of this! The full name is QuickAssist and it does encryption too. They advertise 100Gb/s symmetric crypto, 70Gb/s compression (or roughly 100x faster than ZStandard on a single CPU). Seems to retail for about $650 for a card.

It seems like QuickAssist can also be used for Transcoding. Plex e.g. supports it.

My Intel i9 11900k seems to support it or has it built-in.

QAT is actually likely to end up inside new server CPUs from Intel - at least according to the advertising material. Also, it is in their new SmartNICs. At least somebody is using it.

It has been done before, if you offload gzip you turn your PCI bus into a choke point. Most of the time you do better keeping it on the main processor.