Apple is not responsible for the Wiris internal product planning and delivery schedule for MathType, which after the acquisition from Design Science, is now a subscription product that still hasn't been ported to Catalina or Big Sur.
As it currently stands you can subscribe to MathType for use on Windows 10/MS Office, Google Docs, and Word for iPad. I recommend you contact Wiris and see what their response is to Catalina or Big Sur support.
What happens if Wiris chooses to not support MathType on Big Sur or later. Does that mean that you would remain on Mojave indefinitely rather than adapt to a different equation solution, such as the built-in equation editor in Pages?