Possible "custom NBT"

I took a command block and typed in a summon command, as I get to NBT I typed things and nothing happaned (its not on the screenshot), so I checked a lot and I did not see any error. I thought that this tag changed on 1.13 but the command block didn't say nothing and the NBT wasn't red.

I am not sure if it's a bug but also if not, fix this because I can't learn in my head any tag...