Comment 1:

Labels changed: added repo-main, release-none.

Comment 2:

These need to be evaluated on a case-bv-case basis, so changing the subject.
At least in this case if you really care about the difference between too large and
other errors, you can simply count the incoming bytes yourself.
The real magic of MaxBytesReader is that it lets you not only stop reading (like an
io.LimitReader), but also marks the underlying http connection as dead, so it's not
reused in the next request. This mattered in more before Go 1.2 when it would always try
to advance to the next request if your handler didn't reach EOF on its own.  Nowadays
MaxBytesReader is convenient but not really required for anything.  I don't think it's
worth exporting more things in net/http or expanding any documentation for this case.
Nothing in the current code precludes people with special needs from doing what they
need.
If there are other unexported errors, please file separate bugs for them.

Status changed to WontFix.