I think the guidance about running 8 speed cos it'll be easier to find when on the road, etc... is probably a bit out dated. 10 speed tiagra, or 11 speed is likely to be easier to find, just cos it's more common now.
But I think this also ignores to an extent the fact that these days you can get a bike part shipped to almost anywhere in the world in a day or 3. If you can bodge enough to get you to a place that can receive post, you can get the parts you need, tho it may not be cheap. Tho if I was going that far off the beaten track, carrying a spare freehub body, and even the axle kit for a hub isn't gonna slow you down that much.
I know when working in a bike shop in the European capital of cycling, that anything other than the current line of what ever shimano are making, can be pretty hard to get. We certainly didn't have a box of 8 speed cassette just waiting for someone with an old bike came in.
I do like the idea upthread about carrying a friction shifter as a backup, has excellent bodge me to a bike shop potential. But cos I'm on Di2, I can just remap the buttons about to get the shifting I need. I also carry a enough spare wire to recable most of the bike in the event of damage. They weigh very little, so are easy to carry.
J