What program has replaced IDVD for MacOS running Catalina

Apple did not replace iDVD. Check the Mac App Store for something comparable, or outside of the Mac App Store. If outside of the Mac App Store, verify beforehand that the solution is 64-bit compatible with Catalina.

Sadly, nothing exists at the level of iDVD.

In our video lab, we have several Macs "frozen" to OS 10.9.5 so we can use iDVD and iMovie 06 HD.

If your needs are simple, there are alternatives. With iDVD, you can have Hollywood looking menus with very little effort.  Toast would probably be my second choice. But it is a distant second choice. Toast will let you make menus, but these menus will not have motion, and they are set at fixed intervals. We use Toast for making Blu-rays.

Because of the advent of 1000 year lifetime, M-Disc I have seen optical media making a strong come back.