Ghost of Tsushima DIRECTOR'S CUT can be activated in my country and potentially different ones too

Yeah, they dropped their region restrictions some time ago. I don't know for how many games by now but they certainly learned something.

Can you submit the (used) key to SteamDB? Maybe it will update that sub to reflect the relaxed region restrictions?
https://steamdb.info/keys/

The reason is probably, that Steam and resellers are forced by EU law to sell any game, that is being sold in at least one EU country, in every EU country (unless there are local laws prohibiting that) and allow cross-border trading within the EU to maintain the EU-wide price competition.

So keys, that exclude or include some but not all EU countries via region restrictions, should work in every EU country nonetheless. Unfortunately this is not necessarily reflected in the region locks configured and shown in the according sub on SteamDB. And people need to make sure to not confuse EU and Europe.

Another reason could be, that your Steam store region may not be set to Latvia, since that is what counts when activating keys, not the current IP address. You can doublecheck here: https://store.steampowered.com/account/

I'm wondering if other countries can activate it too.