"old skool" exchanges uses either FIX (old and really vernose), FAST (binary encoding for FIX) or custom fixed-layout protocols.

Most big USA exchanges uses custom fixed-layout protocols, where each message is described in documentation, but not in machine-readable way. European ones still use FAST.

I didn't seen FIX in the wild for data feeds, but it is used for brokers, to submit orders to exchange (our company didn't do this part, we only consume feeds).

I don't know why, but all Crypto Exchanges use JSON, not protobufs or something like this, and didn't publish any formal schemes.

Fun fact: one crypto exchange put GZIP'ed and base64'ed JSON data into JSON which pushed to websocket, to save bandwidth. IMHO, it is peak of bad design.

there's still ascii FIX floating around on the market data side for a few esoteric venues

FAST is not particularly common in Europe

the large European venues use fixed-width binary encoding (LSE group, Euronext, CBOE Europe)

Eurex uses FAST for sure, but I can be wrong about "common".

Eurex/XETRA EOBI no longer uses FAST. It is about 8 year old t this point, but IIRC some products are still on the older protocol.

Euronext uses SBE specifically.

And msgpack if you want an order of magnitude faster serialization/deserialisation and can put up with worse compression (I think mainly due to schema overhead since protobuf files don't store the schema?)

https://msgpack.org/index.html

Good protobuf vs msgpack comparison: https://medium.com/@hugovs/the-need-for-speed-experimenting-...