I've been using Joplin on many different devices for years now. Recently most of my devices updated to version 3. Then I tried to update my main work computer, the one I really need it for, and it won't update because it's MacOS 10.14.
I don't even know how to roll back my other devices, and I guess it's not worth going on anyway.
Same problem. I'm running OS 10.13.6. No sync. Can't update joplin due to OS. Point is, this box is adequate for my needs....I am forced into obsolescence.
Welcome to the community bongo_x. I thank we need to look for a reason other than “because it's MacOS 10.14”. I am running and syncing v3 happily on Sonoma 14.5 with iOS 17.5 devices. Nothing unusual happened during upgrade.
Same problem here after upgrading (I'm running MacOS 10.13.6).
Opening the latest 3.0.13.dmg, the info.plist sets the minimum MacOS version at 10.15.
To me it looks like the image is built against newer libraries which are not backward compatible.
@bikefright welcome to the forum.
Joplin is built using the Electron framework.
From looking at the release notes it appears that with Joplin v3.0.0, Joplin was moved from Electron v26 to v29. That "jump" included the changes made in Electron v27 where macOS 10.13 & 10.14 were no longer supported by the Electron maintainers. This was because Chromium had dropped support for those older macOS versions.
A similar thing happened to Windows users when Joplin was updated to a version of Electron after v23. In that release the Electron framework stopped supporting Windows 7/8/8.1. Again, this was because Chromium / Chrome had dropped support for those older operating systems when the OS's final extended support period expired.
@dpoulton Thank you for the comprehensive reply (and welcome🙂).
If I had a spare ÂŁ1400, I'd be out to buy a shiny new iMac, but as a home system, the one I have is fine and I just can't justify it ATM. I mean I'd have to settle for a 24 inch screen for a start 
.
There is a (relatively) simple hardware upgrade (graphics card) that would cost a lot less and which would then allow me to shoehorn on a newer MacOS....maybe I'll have to revisit that plan 
Same issue here. My question is why v3 being release stops v2.x clients synconising. I'm using nextcloud as webdav synconisation backend. Why can't people be given time to upgrade there clients rather than being forced to upgrade them when v3 is released?
Also if Electron v29 is just used for compliation and any new features aren't being used. Why can't joplin v3 be compiled in Electron v26 to produce a legacy support version. Or even Electron v23 so that Windows 7/8/8.1 users can still use it.
Just found a work round:
Joplin will then sync all your notes again and not produce any upgrade to v3.0 to sync messages.
I guess if you keep updating other sync clients to same webdav repository you might end up with conflicts / errors do to incompatibility in sync structure.
I think that this came about because the v3 clients introduced the trash / recycle bin feature that many had requested. This meant that v3 clients would sync data to the sync target that v2 clients could not use / understand. Whether that was guaranteed to cause problems or could possibly cause problems I do not know. Anyway the version requirement was added. Whether that could have been done better; possibly. However for those with a current OS it would just be a case of upgrading the client.
What?? I don't think that is the case.
Joplin is built with the Electron framework, not by it. Electron / Chromium is a part of Joplin. Later releases of Electron include improvements, bug fixes and, importantly, chromium browser engine updates and CVE patches.