Macbook Pro
Ventura 13.2.1
Joplin 2.9.17
Are there any plans to enable outlining in Joplin? I've been using the 'Bike' app and finding it extremely useful. Would this be possible with a plugin? Sorry, I'm not a techie so I have no idea, but in my opinion it would certainly make Joplin even better than it is already!
There is an "Outline" plugin which may possibly be of use to you? I have not tried it (or "Bike") myself so I do not know how they compare.
It can be installed by going to Tools > Options > Plugins and searching for "outline".
Do you mean like moving bullet lines? In markdown mode you can use Alt (option)+Up/Down So far that is as close as I have seen to some outliners
What I really would like is an outliner like Folding Text, with the ability to focus in (zoom) on areas whilst hiding (folding) the rest. So far I've not been able to do that with Joplin. I don't think the 'Outline' plugin is quite what I need either. Check out the free version of 'Bike' if you have Mac, which does exactly what I need, or Workflowy or Little Outliner which are web based. Here is a screenshot from Little Outliner:
I think outlining generally refers to things like being able to easily reorganize note contents, chapters, etc. Can be useful when drafting longer texts, etc.
edit: ah, beat me to it.
I would also love the ability to hide/unhide text in an outline by clicking on an arrow. I used to have a plugin in MS Word that did this. I know that Joplin can't do everything that everyone wants, but if I had to vote for a new feature (either built-in or as a plugin), this would be very high on the list.
Outlining does seem to be possible in Markdown. I found this app... (Outline Markdown Reference | Markdown Guide)
Sure it's possible, we're asking if it's practical in Joplin. 
I use Typora for external editing, and it has an open feature request for drag-drop reordering of sections; which would basically cover most of my needs. It's been open for 6 years though, so I'm not holding my breath. 
Expanding and Collapsing text under headers is possible in the Markdown Editor with the Folding in Code Mirror Editor plugin by @ambrt. The folded state of a note is persistent and will be remembered even after a restart of Joplin. It might be of interest.
The details element can handle simple collapsing/expanding natively. You have to write the HTML yourself, but the basic functionality is there.
See: <details>: The Details disclosure element - HTML: HyperText Markup Language | MDN
<details>
<summary>Expand me</summary>
Expanded words (including <em>other</em> HTML) go here.
</details>
Not ideal, but it can be a stopgap until someone comes up with a plugin for it or Joplin gets folding built into it.
Maybe an external editor like VsCode/VsCodium could do the trick. I know folding by headlines is possible but not sure about moving around sections. There's probably a plugin for it if not possible natively.
I use the Folding in Code Mirror Editor as mentioned above, but sometimes find opening a note shows everything open even if some were closed (indicated with the period at the end) - this may be because I am also using Rich Markdown - in any case, if I switch to another note and close it again, it works fine.
Very happy to hear about this Outline plugin though, that's exactly what I need to add to my workflow, but hadn't thought to look for!