I do honestly believe that Kidge was off screen canon, like, to me it's kinda obvious? When Keith lost it, who was the only person that he didn't get snappy with and didn't get snappy with him? Pidge. That alone says a lot. Pidge and Hunk both grabbed him, but he only got upset with Hunk, infact Pidge's touch seemed to calm him down. When they got dragged under, while yes Pidge was the closest to him, he held a death grip on her, just her, instead of trying to help anyone else too.
I believe that around Season 3, they were unofficially canon, because around that time, the atmosphere was different between them, they were a lot closer, we saw that in season 4 when he left the team and he seemed to be the most affected by her words. We especially saw that same atmosphere, although a lot stronger through Season 6-8, especially 8 where they were almost always near eachother.
Here's what I believed happened: Even the people who don't ship Kidge wanted them to know eachother before Voltron so they could bond over the loss of their family. I believe we got that in season 3, it wasn't seen, but they definitely bonded over trying to find their family, and that led to them being extremely close, but I bet they kept their friendship a bit secretive so the team wouldn't tease or ask questions. So when Pidge called Keith the loner, he understood, but it affected him deeply, and it caused tension between them. It went away tho when it was confirmed he would be their leader, and she comforted him and apologized, I figure that because after it was decided, the tension went away. Eventually they caught feelings and wasn't until right before they found "Shiro" did they confess, BUT, the reason why we never saw the relationship between them, was because they kept it secret because they didn't want to mess anything up within the team. It was a secret relationship, that's why it's off screen or unofficially canon. Their was also more tension when Keith went after Acxa, and it's also why he's more upset when they said "I guess it is true love" he wasn't embarrassed, he was annoyed because he has no romantic feelings for Acxa. Obviously Pidge doesn't know that so it caused tension between them until they fully talked it out, and I'm guessing that's where the lunch date came to play. They kept it a secret until after the war was over, and it was confirmed with the Keith & Pidge mice forming a heart.
Bonuses: I think the team started to catch on around season 6 when Keith got back, that's why when they were talking about where Kosmo would go, everyone looked at her, it wasn't just because she has a dog back home, but because it would be best to have the girlfriend watch over her boyfriends dog...erm sorry wolf. Plus Lance had Katie & Keith team up when Lotors troops broke in the ship, knowing they worked well together.
Also keith's never been one for physical touch in the team but easily welcomed Pidge's whenever she hugged or helped him.
Plus let's I don't think I even need to mention the obvious tension ever since the incident in season 1.
