Seems like a commit that was done yesterday prevents any VST3 plugins (even just blank template ones) from being scanned by hosts. In some cases if the plugin was scanned from a version that worked, it would show up but not launch.
I tried it on Intel Mac/Big Sur, on Ableton Live and PluginVal. VST2 seems to be fine, as well as standalone.
The problem seems to be related to this commit:
Anything before that runs fine and does not show the error message.
The actual error that shows up is:
error: Failed to load plugin: bundle does not export the required 'bundleEntry' function