Clickable stick and + / - button
True. Rocket League doesn't use clickable sticks, but +/- could be used for menus.
6 buttons isn't even enough to cover the essentials.
Boost
Jump
Drift
Accelerate
Brake
Change camera (it is essential)
Menu
Then you also have he scoreboard, the quick chats (which aren't essential) and the second stick for rotating the camera, which is still important even with the ball cam on. The game would just not be fun on a single joy con, coming from someone who has nearly 400 hours played.
Aside from the menu button (which can be mapped to +/-), the only essential controls are one stick and five buttons (forward, reverse, jump, boost and powerslide). You may prefer to have a second stick for camera control, buttons for scoreboard, etc, but they're not essential, and the game would be perfectly playable on one joycon. There's certainly no reason whatsoever to prevent people playing with one joycon, particularly if it facilitates local multiplayer.
It also has two clicable sticks, - or + buttons, Z/ZR or L/ZL which could be used for secundary actions with proper combination. There are other alternatives, like the buttons + or - could be used for scoreboard if you hold the button, but if you press it really fast it could bring the Options menu.
We were talking about using a single joycon, not a pair.
Have you ever played Rocket League? there is physically no chance to use the current control scheme on a single joy con
Well over a hundred hours worth. Have you ever used a joycon?