Unity already supports the Switch, but like most consoles from the big three (Microsoft, Sony, and Nintendo), you need their permission to be able to develop for it. You need to register an account with Nintendo's developer portal and then contact them through that to start the process.
If you want to develop for the Switch, you cannot use a retail unit. You will need a dev kit. Here are links for getting started with console development on the three major platforms: http://www.xbox.com/en-US/developers/id https://www.playstation.com/en-us/develop/ https://developer.nintendo.com/
^ This covers it. Discussions about developing for these platforms is typically under NDA and there are specific places to discuss it once you have been approved as a developer. As for the intellivision, they have said that they have partnership with Unity, and it will be supported. But it is still a long ways out for launch (if it actually happens). If it does, it will likely be the same set up, you will need to work through Intellivision to get the SDK.
My nostalgia has me very tempted to buy one of these. https://gizmodo.com/a-commodore-64-clone-with-a-working-retro-keyboard-will-1835836875
This is really more akin to the NES/SNES/Genesis/PSX (hopefully not)/Turbografx Minis, but a little beefier since having a single purpose device of that profile is a bit of a mess. A far more coherent idea than things like the Amico, VCS, and that other one I forget.
Thread is a bit little stale but, Unity currently supports the Amico. According to Tommy Tallirico 90% of the launch games are being developed in Unity. The platform is Linux/Android so not a huge stretch for Unity to support.