To judge from past discussions in this forum, a decent number of Scrivener users have relied on MathType to add equations and formulas. A year or so back Wiris took over the development of MathType from Design Science. The other day Wiris wrote owners of their newest version (like me) warning us that no 64 bit version of MathType will be ready when Catalina is released; no existing versions of MathType will run under Catalina. So anyone who needs MathType for day to day work should delay upgrading to Catalina for the time being. Wiris isn’t yet sure how long it will take to finish a compatible version.
A competing product with a very similar interface I’ve occasionally used in the past is MathMagic. But the last time I contacted them they had no plans for a 64-bit version.