What Apple TV generations support airplay and surround sound through airplay?

Does that mean Generations 3 and earlier support airplay 1 or original only?

Yes. The 2nd and 3rd generation model support original AirPlay 1 (not numbered back then).

AirPlay system requirements - Apple Support

An older version of this document was more detailed on differences between original AirPlay vs. AirPlay 2.

I am wanting to connect my Sonos surround sound system via airplay through the Apple TV.

That won’t work. See below.

I know Airplay 2 supports surround sound

It’s a bit more intricate than that. Users can AirPlay-stream video with surround sound to Apple TV, but streaming audio from Apple TV to AirPlay speakers will be in stereo.

For surround sound, you will want a wired connection. Depending on your setup, you may need an audio extractor adapter.

Hi, thanks for this answer.

I was aiming at the exact same setup as OP: an Apple TV 4k plugged to a projector, streaming (hopefully surround) sound to Sonos (Beam Gen 2 soundbar and speakers) via Airplay 2.

It is disappointing if audio streaming from Apple TV only allows stereo then... Where can we read about this? I can't find any information about the supported audio formats of Apple TV 4k and Airplay 2 anywhere...

Is there any way you would suggest for achieving a similar result: wireless surround sound (to Sonos or others) from a source like the Apple TV 4k?

The only exception is the large HomePod speakers, which can virtualize surround sound (still two speaker units after all).