Someone could be wanting to temporarily hide a list or multiple steps of logic that requires multiple lines. Neither of these is compatible with the abbr tag.
As for multiple lines, you can just use multiple tags. Simple enough. If you need the list to be vertical for some reason, there's surely a different solution than shoehorning a spoiler alternative into our (very) limited BBCode capabilities.
AFAICT, none of the tools available grant us a one-page spoiler
button. You could link someone to a separate page or an anchor link to a separate post on the topic if you wanted something more involved.
My point is that someone structuring a post should keep in mind that at a certain point, it seems dubious to hide a significant amount of content behind a spoiler tag. If it's important, you should put it on display in your post. If the point is to save space, then we have multiple posts, URLs, and anchors.
An indexed post similar to
this thread also achieves a similar "click to see" structure.
Granted, I would not be against implementing the spoiler tag. We do have very functional alternatives, though, and the addition is only really for accessibility purposes.