By that, I'm not asking whether the indigenous knowledge of today will be preserved decades into the future. Though that's an interesting question
in its own right.
But even given a radical reversal in the trend of communication and transportation costs, I don't see new groups becoming culturally isolated in the near future. Nor do I see the groups that are currently somewhat isolated remaining so. (And given a far-future scenario where civilization falls and the regions of the world are again isolated, who could acc**ulate the resources
to be the next colonizing group?) One of the strongest cultural barriers, that of groups not knowing of
one another's existence, is far more easily broken than reestablished. And if there are still secret tribes, they are few in number.
Isolation carries benefits, but also significant risks. Isolate yourself, and you guarantee that you'll only run into people with the means to overcome whatever barriers kept your group isolated and no reason to like you or your fellows.
So to answer what local secret I'd pass on, I say: "Local? Secret?
It's on my website!
(I suppose interstellar distances are still a significant enough
barrier to allow for cultural isolation, but I don't expect humans will
be crossing them within my lifetime. And I wouldn't count on extra-terrestrials showing up and making all of us "indigenous", or on them being kind to us if they did. I don't count that as a likely scenario, and all bets are off in that case anyways.)