That was... unexpected.
http://www.nintendoworldreport.com/news/49693/pokemon-lets-go-demo-now-available
If you're still on the fence about Pokémon Let's Go, you can now make the call for yourself with a demo.
The demo version of the November release is available now from the eShop, or the game's page on NIntendo's online listing, and is available worldwide.
It appears the demo is based on the E3 2018 demo that sets the player in Viridian Forest, with a random choice between Pikachu and Eevee. Pokémon catching is available, but the data does not transfer to the full game.
Interesting that they would introduce this now. Is it not selling well? To release a demo this far after release suggests that Pokémon fans are iffy about this "not quite proper" Pokémon game so Nintendo is hoping to win them over by letting them try it for free.
I think it's a good idea but is probably should have been the idea from the beginning. I was always turned off by what appeared to me as a simplified take on Pokémon but if I owned a Switch I would totally try it. My brother is a big Pokémon fan and he has a Switch so I'll suggest he try it.
Nintendo has traditionally had an aversion to demos because they seem to think people will settle for it rather than buy the game. There are a lot of games that I've bought because I liked the demo and I think that's typical. Now I've also decided to not buy games I was unsure of because the demo confirmed that the game wasn't for me but it's going to go both ways. The alternative is I just never try the game at all and it's a lost sale 100% of the time.