If I had to guess, I'd say The Witness isn't coming to Switch. Creator Jonathan Blow has been asked about it numerous times now, and he's been wishy-washy on the idea at best. Here he is once again talking about the potential for a Switch port to Eurogamer.
"I wouldn't say never, but we worked on that game for a long time. We could have done more ports but I felt like it was a good idea to give the team a break and work on some new things. We're working on new projects now. We're not really working on ports of The Witness but that could change. Something could happen. We could outsource it and quality control in-house. But we're not currently doing anything."
Hopefully Blow's new game ends up making its way to Switch. I know he had slight interest in bringing The Witness to Wii U way back in the day, but that never came to be. With Switch's popularity, I'd think supporting the platform with his newest game would make a lot of sense.