At https://tex.stackexchange.com/a/383023/73371, I wrote a short answer
Thanks a lot for reporting this and all your research into that matter. Thanks @egreg! This seems indeed to be a leftover of an experiment I did once and did not completely reverse. And my (wrong) code stopped working with an expl3 update. I will publish an update of mhchem during the next couple of days.
I got a delete vote and a comment I should neither say thanks nor announce a future update.
What's your policy on a package author saying "I am working on this"?
As a package user, I would find it helpful. (The participants were talking about contacting the author, anyway.)
What's your policy on announcing upcoming updates?
As a package user, I would find it helpful.
What's your policy on a package author explaining a bug.
I would find this interesting.
What's your policy on saying thanks along the way (not as the only part of a message)?
The question will be closed anyway after the update, I suppose.