The easier fix here is to avoid level 2 headings unless you are starting a new discussion , e.g. by using level 3 instead

I resorted to just use nowikis, but I would call both ways workarounds. While it has been kind of known that collapsed sections are still sections (they do end up in TOCs, after all), I think users do not expect something they put inside a collapsed template to affect the subscribe link visibility.

In that discussion the collapsed part was used to propose the text of a new guideline. Sort of mentioning the text without it taking too much space. I agree this may not not ideal (I would personally put the wikitext inside <pre> tags) or the intended purpose of mw-collapsible-content, but this happens quite enough in discussions, at least on itwiki.

Wrapping the collapsed fragment in <div class="mw-notalk">...</div> or in <blockquote>...</blockquote> (as documented at https://www.mediawiki.org/wiki/Help:DiscussionTools/Magic_words_and_markup) would also make the subscribe button and discussion metadata work as intended.

However, the table of contents and section edit links would still behave in a very confusing way, so I don't recommend that in this case. The nowikis work better.

On English Wikipedia, they have a template for this situation: https://en.wikipedia.org/wiki/Template:Fake_heading. There's also a request for some way to do that in MediaWiki itself in T8575.