Firstly, I recommend that you use a tool such as bikeshed to generate
HTML proposals. This proposal could be a lot prettier.

Also, why is the abstract much longer than the introduction?

The motivation does not explain at all why std::contains is necessary,
compared to std::ranges::contains and container.contains()

It doesn't help your proposal if you make unsubstantiated claims like
"optimal implementation", "offering performance and stability
benefits". This is unsubstantiated fluff and would get torn
scrutinized by the committee. Everything that isn't obvious needs to
be justified.

I remember that you've previously discussed these functions as
customization points, but the reference implementation doesn't make
use of .contains() at all. I'm assuming that this is more of a mistake
in your reference implementation, otherwise this proposal is
completely pointless because std::ranges::contains already exists.

If you don't have any proposed wording, just leave it blank or write
"Work in progress." It doesn't help the reader if you pad these
sections out.