Developers versus administrators

If there wasn't an implicit backwards-compatibility guarantee for stuff in debugfs, people could simply move the "production-ready" parts like perf to a final place in sysfs without any hitch. Yes, they would be gone in debugfs, but you're not supposed to depend on anything there.

Actually, I don't see the big problem. If you deem /debugfs/foo/{bar,baz} such production interfaces, why not move them to /sysfs/.../foo/{bar,baz} and provide the compatibility via a symlink? Is that really a maintenance burden?

I'd really like a mirror policy for phasing out ABI. I.e. if an interface has been in the kernel for X releases and Y years when it goes on the deprecation list, it can be removed after another X releases or Y years (whichever is greater).