No, it’s a new limitation of Mojave.
Sorry, I was confused: Cmd-Shift-4 gives access to Markdown with Mojave, which was not the case with Sierra. I thought that only Cmd-Shift-5 gave access to Markdown.
thank you for your comments!
Hello Adam,
and thanks for your reply!
I agree that it wasn’t a good idea to postpone, but I decided it was either now or no more at all… It is mainly the ability to keep receiving security updates that gave the last push. But I’m seriously wondering whether I’ll go 64k-only in the near future.
To come to your second question: occasionally I use vMac to run legacy software (current versions of Microsoft Word can no longer read my old files, for instance) and this still works fine and has no problems reading any of these disk images.
Best, Minus
Ah! I understand what you’re saying now. It’s not so much that Mojave doesn’t allow you to add sharing extensions (though that’s true), it’s that Apple dropped system-wide support for Twitter, Facebook, LinkedIn, and Flickr. You can get Twitter support back with Twitterrific.
Since Facebook doesn’t see fit to produce a Mac app, there is no app that can provide a sharing extension. Think of it as Apple protecting your privacy and making the world a better place by making it harder for you to give more of your data to Facebook for exploitation. 
Ah! Once again, I think I understand what you’re saying now—you mean Markup, not Markdown. Markup is Apple’s technology for editing screenshots; Markdown is a language for applying styles and structure to text.
Markup is available in a variety of places in Mojave, including the Finder.
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yes, Markup Thanks for the tip about Finder+Markup
I agree. It’s definitely much improved.
The one thing I really miss now though is the ability to choose a name for the file to be saved. You can edit almost any aspect of a screen shot in its preview window, but the one thing I cannot figure out is how to use that panel to set a name so I don’t end up with yet another file generically called Screen Shot 2019-07-…