I think my answer is that it depends (helpful, I know).
Lots of this is simply rehashed from this topic but with some elaboration.
On the topic of the feature itself:
When I use something like Typora or Mark Text I tend to have the application as a floating window so the width of the window tends to be small therefore the "margins" tend to also be small. Also when I'm using one of them it is more like I'm writing a standalone document vs one of many note within a hierarchy so I don't really require anything more than the window itself (i.e. I don't use a sidebar) so it never feels like I have any awkward wasted space on the screen.
When I use Joplin I mostly use it on an ultrawide monitor, maximised and in markdown editor/viewer dual pane which, on the default 600 editor width, leaves some fairly substantial unused screen estate and looks really quite awkward. It also means my eyes have to travel much further between my sidebars and my note content which is surprisingly taxing.
One of the benefits of markdown is that I don't have to care about my page width like in a word processor, I enjoy the fluidity where I can make a page as wide or as narrow as I want without caring about how it will affect anything.
I also very much subscribe to the idea of "markdown is code" approach so ultimately if a text editor like VSCode or Atom doesn't enforce this by default, I wouldn't want my markdown editor to do it either (although I understand there are several perspectives on this which have been discussed at length in the syntax highlighting change discussion).
However where I can see this being particularly useful would be on the richtext editor and the single pane markdown viewer. I see the richtext editor more like a word processor and so a "page" would feel a little more natural, I think there were previous suggestions about maybe including some kind of visual hint to mark the boundaries.
I do think an additional justification option would be good here as I believe having a maximum width but left justified may be fairly popular.
On the topic of the implementation:
The biggest complaint that I can totally understand the frustration is that this change along with the codemirror syntax highlighting change are two of the only changes I can think of recently that have directly altered the user experience - positive for some people and negative for others. Whilst I appreciate there has often been RFC posts about changes like these I think many people either didn't understand the potential impact or didn't assume that it would override the default behaviour.
Related to this I also feel that the changelog is often a little too concise for many of the changes being made - its fine as a github summary but not ideal for a user who may not be up to speed with discussions on the forum.
The change log line for 2.3.5 was
Improved: Allow setting a max width for the editor content
which to me implies that a setting has been added for a user to enable this (not that I think it is overly clear why one would want to do such a thing based on the description alone), not that it would be applied for all users after the upgrade.
What doesn't help is that I think the wording "Editor maximum width" is somewhat ambiguous in its wording. It doesn't explain exactly what the number represents and the fact is that many people don't always use the full width of the editor for notes so it mostly manifests itself as a "left margin" rather than a centred "page" which leads people to look for "justification", "margin" or similar as an option.
So in summary or as a tl;dr
I think it is mostly fine as-is for the richtext editor and markdown viewer but not for the markdown editor (although I understand this opinion will be contentious) - therefore maybe it needs to be considered "per editor"
I think an additional justification option would be popular
I think the wording in the setting itself could be clearer as to its function
I think that major UI or functionality changes that alter the default behaviour of the application either need to be very clear to the user on upgrade (like using the dismissible banners) about what the changes are and how to regain the original behaviour (if applicable) or should be available by choice initially with a warning that the functionality will be default in the next major release.
Edit - It looks like they added the feature to Evernote at some point too. Its not as customisable but I think its a lot more user friendly