I also support this. I'll work on it as soon as possible, but I likely won't have time for it until Saturday.
My plan is to implement this as a [collapse] tag. It expresses the purpose just as clearly and we'd keep backwards compatibility with posts using the old [spoiler] tag.
Speaking of the current spoilers, are they still broken on iOS? I have absolutely no way to test them and I'm not sure why they don't work, but it would still be nice to know. Anyhow, this isn't related to this issue.
Collapsibles have got the advantage that you can hide a large block of a text which might be useful for a larger code without using pastebin and similar sites. Another advantage is that you can pack multiple collapsibles inside another which doesn't work as much with the hover spoiler because the latter reveals every spoiler which you can avoid with the button.
Which also means, colappsibles should have the possibility to attach a name and uses by default "show" or "collapse" and "hide".
At first, I got lazily practical and tried using a <details> element for this. Then I realized that our dear lord and savior Microsoft Edge (and Internet Explorer, which the site is also supposed to support) doesn't support it, so that was the end of that idea. Then I realized the HTML filter doesn't support IDs on elements - and for good reason, so a conventional checkbox hack wouldn't work, either.
This issue is now resolved. It will be closed if nobody replies with the feedback or complaints in the next few days.
Anyway, nobody has complained about this so far, so I'm closing the issue for now. If you have more feedback, feel free to contact me.