...if one even exists?

Get the Demo 2.0 instead, there're no Steam keys for a 20 years old game, do you think in 100 years they still are going to have keys for it?
Here you can connect to the only server available to Demo and play with others:
https://steamcommunity.com/app/10000/discussions/0/599663354106173175

There is only 1 Steam gift in the existence and the seller expects you to pay 5 digits...
Besides, authentication server shut down in 2021, rendered creation and/or usage of online accounts impossible. Use Makron Horde ETQW Revival to play the game instead.
https://discordapp.com/invite/tX7Buk9
https://etqw-stat.herokuapp.com/overview

Retail copies are pretty cheap.

Do not connect to the TAW / Makrdon Horde servers, they are hackers and distribute malware by their own admission, see these screenshots:

https://imgur.com/a/YxEWfmy
Compare their files to the official checksums on
Splashdamage.com

Do not get infected from TAW.
You do NOT need any hacks or cracks to play, and you should NEVER, EVER have to download files in-game when connecting to real vanilla servers.
You can get clean files on
ETQW.org
that match the checksums posted on Splashdamage. They have clean servers you can connect to and games every day. Stay safe.

There is only 1 Steam gift in the existence and the seller expects you to pay 5 digits...
Besides, authentication server shut down in 2021, rendered creation and/or usage of online accounts impossible. Use Makron Horde ETQW Revival to play the game instead.
https://discordapp.com/invite/tX7Buk9
https://etqw-stat.herokuapp.com/overview

ahhh THATS why I couldnt use my old cd-key when I installed it on my new PC last year, good thing it works just fine without it as long as you dont wanna play online.