Firefox currently shows a red crossed out padlock for HTTP sites with form elements, but not yet for HTTP sites without form elements which for now get neutral treatment. The rationale is that you definitely shouldn't be using insecure forms, what could you possibly be writing where you really don't care about at least confidentiality (to prevent eavesdroppers from reading it) or integrity (to prevent a MitM from changing it) ?
If you set HSTS and then subsequently remove HTTPS from a site it should (will for Firefox, kind of for Chrome) brick wall you, saying that it isn't able to reach the HTTPS site without offering to let you see the insecure and perhaps compromised HTTP site even if you spell out the HTTP URL.
Unlike HPKP this isn't considered a foot gun because you can fix it by just enabling HTTPS, and why didn't you have HTTPS anyway?
The biggest forward pressure for HTTPS is that newer protocol versions (after HTTP/1.1) do not in practice exist for plain HTTP. The way to do plain HTTP/2 is documented but nobody has plans to implement it, and there isn't even intent to document a plain HTTP/3 because the stuff it's built on is all encrypted from the ground up. From my point of view this is good news.