Not GP, but we do the same. Branches become the atomic unit of bisection in master, but the need is extremely rare. I think because we have good tests.

We also keep merged branches around. This has never happened, but if we needed to bisect at the merged-branch level, we could do that.

I know squash-merge isn't everyone's cup of tea, but I find it to be simpler and clearer for the 99+% case, and only slightly less convenient for the remainder.

The range reason your history textbook is not infinitely long. The longer something is, the less digestible. When we need more granularity, it's there in the branches.