I think i saw this kind of description in a game release topic but can't remember where .

Shroud of the Avatar?

No . The game that i'm looking for is already in the Gog catalog .

I'm not sure you'd easily find this. It's an obscure comment in a release topic? I mean, there's alot of games that this could possibly be describing, but might not be so in the same way that you're thinking. Toss us a bone and give us a little more to go on.

Balrum?

Didn't ultima online have respawning enemies, though?

It was an MMORPG so yes, monsters and animals respawned. I loved that game in its first few years of existence. Unfortunately, near enough all subsequent MMOs followed the Everquest template rather than UO.

I 'm not familiar with Everquest, and i'm only aware of Ultima Online from an interview about game design lessons, so i'd like more details.

Could it be Nox Archaist you are looking for?

Maybe CrossCode?
It feels like an Online game, while it's singleplayer offline.

https://www.gog.com/forum/general/in_development_crosscode_e1f14?search=ultima

Just based on the graphics style, could this maybe be Ember?

Ultima Online was a true rpg in that you could play your character any way you wanted. There were monsters and dungeons if you wanted to do the usual fighting/ looting thing but a lot of people created craftsmen. I had the second blacksmith to reach GrandMaster status on the Europa server and had people literally queuing up to buy armour from me with my name on it. Reaching GM level was a big deal in the early years of UO as it took a lot of grinding, particularly for craftsmen.

You could also buy house deeds and then place them in the world. Because UO wasn't instanced like MMOs today, everyone could see your house and some people used to place vendors on them to sell stuff they had made or found.

Towns were safe but once you went out of their boundaries of protection it became possible for other players to either steal from you or attack you. Those who specialised in attacking other players names would turn red and they could no longer enter towns without being attacked by the guards. These were known as player-killers (PKs) and it was great fun getting posses together to go hunting them.

Could it be 'Serpent in the Staglands'? I haven't played it but somehow it has always given me Ultima Online vibes.

Man, Doc's comment takes me back, I loved UO and I remember seeing it as a kid for the first time and not believing all those characters on the screen were other players. It really was an awesome ride despite all the grinding, exploring was risky and visiting the cemetery to level up was terrifying. I played a tailor mage and those spell sound effects are forever engraved in my mind.

Another possibility: Kingdoms of Amalur: Reckoning, I am not sure if it has been compared to Ultima Online specifically, but several websites have compared it to a single-player MMO.

Sounds a bit like the game i want to build. There'll be an obvious benefit to combat, but you can otherwise just do whatever you want and succeed that way, be it farming, prostitution, crafting, looting, slavery, whatever.