I'm a MacOSX plugin developer, and in touch with others also facing this situation.
I develop on a VERY old machine in order to support backward compatibility way way farther back than Apple will allow: my current plugins will run on PPC machines because those can be used as music DAWs. As such, the machine I'm compiling on is not producing 64-bit AUs that will work, directly, on MI Macs. They work on literally everything up to that point, but Apple finally shanked me, at least w.r.t that compile target. Until then I was able to support PPC to present day with one three-target fat binary :)
Another dev, Sean Costello, told me that older builds of his stuff (pre-2017?) weren't running on M1, but everything built past a certain point (a new version of XCode, that had long abandoned things like PPC and possibly 32-bit support) was automatically working on M1 through the Rosetta layer.
So, depending on the build environment, Apple arranged that the audio plugins don't even have to be updated. Depending on the libraries the plugins rely on (a vulnerability for some of the big names that use bespoke but OLD libraries to do things), some of the plugins might need only a recompile to be native to M1 architecture. And some might be really intractable.
The new Apple Silicon chips might be a lot faster. But I am pretty sure that audio plugins take a long time to update.