Just to add - of course they need to be sheered every year, and need other attention, would have the same issues with surviving long term without a shepherd like any domestic sheep - but as well as that an additional issue - hill sheep also have a “culture” of sorts. They learn from older generations where are the best places to go, for shelter, food etc.

Sheep on the slopes below Dunion Hill (C) Oliver Dixon - these sheep remember where are the best places to go on the hills and pass on this sheep “culture” to later generations

This was a big problem in the border areas of Scotland when many sheep herds were killed in their entirety due to foot and mouth disease and this means the new sheep had to learn the hills all over again.

Sheep have a good memory - they remember flock members years after separation for instance. Animal behaviour and welfare: Sheep Part 1

So they do also learn from each other too. They can’t manage by themselves very well without a shepherd, but for hill sheep, the shepherd also relies on the sheep too to pass on knowledge from one generation to the next.

I see this has just gone to the top of the page of answers. Didn’t expect that - it’s one of the side effects of having a fair number of followers. I’m not a shepherd. Just live in a rural area and have a sister who keeps sheep and another sister in the borders who has shepherd friends who told me about the Borders issue with hill sheep after the foot and mouth outbreak.

Please read the answer by Sabine Ehlers' answer to Can a sheep live without a shepherd? who is a shepherd. and Quora User's answer to Can a sheep live without a shepherd?