Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of
text editors" <bug-gnu-emacs@gnu.org> writes:

>> I really don't understand this code, I'm afraid.
>
> Don't worry: I don't think anybody does (I sure don't).
>
> It's code inherited from *many* years ago, where it approximated the
> desired semantics via approximate hacks (like delaying assignments
> until some package is loaded).
>
> We could try and go back to the CLHS doc to figure out what it "should"
> do, and then implement it better.  But seeing how little it's used,
> I doubt it's worth the trouble (my impression is that its intended
> semantics doesn't align very well with our implementation(s), so it
> would take work and/or ugly hacks to make it work "right").
>
>> So I'll commit the minimal change above, and hope that the rest of the
>> problem goes away when you have the time to look at it next :-)
>
> Looking at it doesn't make me very happy, so I don't have any plan to
> look at it.
>
> If you want to do me a favor you could try and mark it (both `proclaim`
> and `declaim`) as deprecated.

(This is a spin-off from Bug#63288.)

The code for the cl-lib declarations, i.e. (info "(cl) Declarations"),
is hacky, hard to maintain, and not well-understood by anyone.  It
doesn't seem to provide any benefits over using standard Emacs Lisp
facilities such as `declare`, `defvar`, and setting byte-compiler
variables.

The attached patch marks cl-proclaim, cl-declaim, and cl-declare as
obsolete.