There have been a few discussions about MPE: MIDI Polyphonic Expression which are spread across different Discourse topics so I thought it might be a good idea to bring them together here. (@jofemodo could you move the relevant messages from Zynth club topic to here?)
MPE is a mechanism to allow expressive control of MIDI instruments using a MIDI channel per note then allowing any MIDI CC to affect just that note. It round-robins the channels dynamically hence MIDI 1.0 allows maximum 16 note polyphony (16 MIDI channels available) on a single MIDI connection. (There may be ways to expand this by bonding MIDI connections but it could prove challenging to play expressively suing more than 16 simultaneous notes.)
One idea for MPE is to allow (new) physical interaction with electronic musical instruments, e.g. the LinnStrument allows control similar to a guitarist where you have access to 8 rows of notes (like strings) divided into semitones (or whatever) with background colours giving some registration guidance and ability to perform multiple dimensions of control; X/Y/Z axis.
Zynthian does not currently have specific MPE compatibility but may provide some MPE compatible configurations, e.g. Surge in Stage mode. It may be beneficial to add specific MPE configuration which might allow better interfacing and playability.