The MPE presets added in Cubase 10 are missing from the Steinberg Download Assistant for some reason, but they’re available from the Padshop and Retrologue content ISOs provided by Steinberg here.
I’ve uploaded the MPE preset files you’re missing below:
MPE_Sounds_Padshop_Retrologue.zip (340.4 KB)
Retrologue and Padshop are based on Halion technology, and will continue to be updated and included with Cubase for the foreseeable future. You’re thinking of the legacy VST instruments Mystic, Prologue, Specter, first included with Cubase 4 back in 2006. These 3 plugins were made in cooperation with a third party developer, and unfortunately couldn’t be ported to Apple silicon.
If you’re on Windows, you can still restore them if you have a previous version of Cubase installed by following instructions posted by other users in this forum. Needless to say, doing this is not officially supported by Steinberg.