@pluto Bitwig does that, kinda. The engine is its own process and plugins go together with it, separate from the UI and session manager. This allows to protect against engine/plugin crashes. It's not really sandboxing though.

For true sandboxing flathub with audio plugins kinda does that right now, by loading plugins within that same sandbox. But it's quite inconvenient, apps from flathub do not see global/system-wide installed plugins (and if they do they break sandbox rules)