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.