This feature request (and it being declined) is almost 6 years old. I'd appreciate if this could be reconsidered. There are a lot of use cases that have been mentioned but I don't even think this should be focused on code blocks. Since code blocks get a scrollbar anyway you can already fill your 30k character limit with a single code block, without taking up a lot of vertical space. So putting in massive amounts of code isn't really discouraged by the system to begin with.
However, I think collapsible regions would be much more useful for normal explanations. In many cases there is the simple answer which will serve most people's needs, and then there's also the in-depth "if you really want to know the details" explanation. In those cases, I would prefer not to take up so much screen real estate for the people who are not interested in the full explanation, but I'd also prefer not to link to the gritty details externally.
You might say "a good answer is tailored to its audience, just pick the short or the long version appropriately" - but I'm not writing the answer only for the OP but also for future visitors, so catering to both cases would certainly improve the usefulness of the answer.
I also don't understand why some people are so strongly opposed to this: collapsible regions are already a thing on SE. On one hand, Stack Snippets can be hidden, but more importantly spoilers are implemented on mobile as collapsible regions. If they're such a bad thing, why have them on one platform in the first place? Couldn't we just turn spoilers into collapsible regions everywhere? (Although a comment-based syntax like Stack Snippets would be more convenient than prepending
>! to every single line.)