An optional 64-bit floating-point mixing engine was added in Cubase 9.5. PG, the main developer of Wavelab, listed the pros and cons in this post:
https://forums.steinberg.net/t/32-bit-or-64-bit/98651/23
The effect of this is extremely subtle, so it’s mostly for peace of mind and for a possible small performance increase.
Cubase 10 introduced the ability to handle and record audio files in 32-bit integer and 64-bit float. This is useful if you’re using an interface that supports 32-bit audio such as Steinberg’s UR24C.
There have been other changes that aren’t directly part of the “engine”, if you want to be technical, but may affect your end results. For example, Cubase now has some of the best sample rate conversion in the DAW market (great if you’re using imported samples and loops), and the Sampler Track now lets you increase the quality of its pitch shifting algorithm. Furthermore, the AudioWarp tools and VariAudio are constantly being improved upon.