It is frustrating that the equation editor in Scrivener is linked to Mathtype which even now does not work in Catalina. Some workarounds have been suggested on the forum, such as learning and inserting raw Latex code, but for an occasional user, that is not appealing. I wondered about other workarounds. Or other equation editors, because moving to a subscription model, as the owners of Mathtype have done, seems unreasonable when the software is still not even capable of running on the latest OS. (My position is that one of my Macs is probably due for replacement, but the new one would come with Catalina pre-installed, and I would be reluctant to downgrade in case that caused problems .
(1) MS Word has an equation editor that works in Catalina, so is there any way of inserting an object created in MS Word, other than cutting and pasting to insert a bit-image graphic of the equation in Scrivener. This works, but is not attractive and the equation is not editable. But I am guessing that the limitation here may be in Word, which presumably restricts you to copying a graphic rather than the underlying object.
(2) If the machine is running Parallels, and a Mojave virtual machine is installed, is it possible to point Scrivener to a copy of Mathtype running in the virtual machine, instead of to a version of Mathtype installed in the host OS. Parallels does allow the user to install shortcuts to apps running in virtual machines, so all Scrivener would need to do would be to point to one of them. Scrivener does not seem to offer the option of allowing the user to tell it where Mathtype is to be found, but I guess there is a configuration file somewhere that could be edited to point to a different place, so it picks up the link to Mathtype in the VM. Of course, that raises the question of whether Mathtype would then save the equation to the right place for Scrivener, so maybe that would not work.
It is possible to cut and paste from MathType in a virtual machine, but that places an image in Scrivener, It works and I guess this is the route I will have to do if I do upgrade to Catalina, but it would be nice for equations to remain editable.