I would like to emphasise the need for this, especially now with Apple Silicon.
AU and AU3 are not just part of Apple's own products, and an integral component of CoreAudio, but they have become part of a larger ecosystem of tools, including Davinci, Audacity, ... If Presonus is supporting AU, which it is since it's an option and it's marketed as such, then it should properly support it. These are not small developers, and they support AU and Studio One ... NI (Kontakt), VSL (Synchron), OT (SINE), SF ... so I see no reason to say that it's not supported.
AU has some advantages over VST3 on MacOS, and the most important is CPU optimisation, on both at play and idle. AU3 is no longer beta according to Apple, at least for Logic Pro, and having. MIDI multi-port AU3 VEP supported in Studio One would be great.
I am disabling VST3 in my template to make sure I have no VST3 plugins, and then re-enabling it so that I can have the VEP VST3 plugin expose multiple MIDI ports. But this kind of workaround should not be required IMHO.