Hi! I was going back to my posted posts after a bit of time, and i realized that one of my post was deleted because it was irrilevant to the site.

Except, it was very similiar to the other post i posted nearly in the same timeframe. I've also checked, and other Ribbon art is infrequently posted, but even Ribbon alone is accepted.

Whom i should contact to get an explanation/undo the deletion/hopefully not get the other post deleted?

That is because posts are handled individyally, on first post it's not clear of what species the male is and artistic style is bit more simplistic. Guidelines also state that skin color does not matter, so they can be considered to be human. On second image it's clear that the second party is far from anything humanoid.

And like said in every single thread like this, which must be reaching hundreds by now, always be in contact with the party who handled deletion/approval in situations like this.

Go message the Janitor who deleted it and ask them.

To me, it's fairly clear that Ribbon has wings, but maybe they see a sign of it being "something else."