So, Is it toxic to open mid?

Yes. The objective is to win. The only exception is to surrender, and the majority of the team has to agree for this to happen. If you open mid, you and only you decide to give up, which is toxic and should get you punished.

Toxic? Nah. Inting and toxicity aren't the same thing.

It always has to be a team decision. Everyone has to agree or it's inting.

Open mid when your team doesn't want you to can get you banned for intentionally losing the game. He more than likely got chat restricted for flaming

If he's just giving up without eveyrone else agreeing to it, then it will be considered as negative attitude and will be punished if he does that over many many games.