What kind of game keys come in hexadecimal format?

tits

maybe some kind of web activation directly in the game's site

That is/was for a beta and you have to activate it on the game's website, then you will receive a steam key for a closed beta. I think it was for Panzer Tactics HD (closed beta which has ended on steam).

Steam accepts some strange looking keys. For example, Dawn of Fantasy keys look exactly like the one that you posted - they don't have the hyphens. Kenshi has like a 50 character long key, double the length of a normal 25 char key.