Yeah, the galaxy is a big place, and the Empire gave whatever other Jedi survivors we don't know about almost 10,000 reasons why they should each find the deepest darkest hole of a planet they can, and just stay very very quiet.
Also as we saw in Rebels; Yoda is not omnipotent. While he could see much from Dagobah, he couldn't seen Kanan until he started back in the Jedi path and set foot inside a temple, Padawan in tow. It's safe to assume that the same applies to the other surviving Jedi who like Kanan, lost their way, or otherwise chose to abandon the path and (unlike Kanan) never return. There may be living people out there who were Jedi, but are Jedi no longer.
This kind of gets back to something I often lament: Star Wars really needs a word that is to Jedi what Ronin is to Samurai. (First person to suggest "Grey Jedi" gets a flick round the earhole and has to wear the dunce cap for today.)
Probably something derived from sanskrit as is Padawan -- which supposedly means "learner" or "follower of the path" depending on where you look it up, but the etymology seems to be literally "foot in forest" or more correctly; "walks in the forest", which I'm given to understand has a heavy metaphorical spiritual overtone to it. So I guess whatever the sanskrit antonym for that is.
I thought maybe "stands (still) in the forest", but that seems to translate to 'Padastha' which seems to be an actual thing in Hinduism and yoga meditation, that doesn't really equate to what I'm looking for here. I thought maybe something derived from either 'Padasaya' ("sleeps in the forest" . . . I think) or 'Paksahara' ("apostate") might work, though that last one may be a bit harsh since it also means "traitor".
Of course it'd take an actual sanskrit scholar to figure out something appropriate. Poking around online dictionaries and glossaries for a language one can't grasp (especially one steeped in religious significance) is probably not the best approach.
Ahsoka, at least at the time, is no Jedi Knight. She's quite easily the next best thing, but she left the Order, and has not come to terms with the idea that she's still a Jedi in all but title. Yoda likely knows she should be Jedi, but respects her wishes enough to no include her as Jedi.
Also, so far as her not being involved in that story is concerned: she can't be the one to do what Luke has to do. She faced Vader already. She can't defeat him. Probably couldn't bring herself to kill him if she did, and she certainly can't turn him back towards the light. Indeed her very presence would be counter-productive as she's just another reminder of his past; a thing he wants to destroy above all else. She is of the past; Luke is of the future. Luke gave him hope. A way forward.
