Thank you for your anwser.
No I didn’t see it.
As I already set full disk acces, I followed the mentioned steps by first closing Mail and DT, removed the mailbundle from the appropriate library folder, restarted DT, installed the Mail add-on again (no longer greyed out), restarted Mail and then did see the manage plugins button.
That’s exactly what happened. But it happened on a device that was competely wiped and used a new installation of macOS Big Sur and DT3.
It’s hard for me to analyse why I couldn’t see the ‘manage plugin’ button in Mail and @cgrunenberg doesn’t appear to have a test device to reproduce what I described to analyse it. I personally wouldn’t be surprised that full disk access is required before the mailbundle is installed and granting acces afterwards doesn’t work unless the bundle is reinstalled.
What I find concerning though, is that other users appear to report such behavior as well as you indicate (‘There is sometimes an issue’). What’s causing that ‘issue’? In the end, it likely means people think they installed the plugin but in fact they haven’t. The mailbundle is there, but Mail doesn’t use it.
IMO informing users with information screens or a text that tells them to ‘enable’ something is confusing if some users are unable to enable it as the ‘manage button’ sometimes isn’t presented by Mail.
If DT cannot itself check if full-disk access was granted like described by @cgrunenberg, nor whether the ‘manage plug-in button’ is presented by Mail (also see my last sentence below), an easy and simple solution might be to provide a pop-up every time the ‘install add-ons’ button on the add-ons dialog is pressed. That pop-up could explain the user has to manually check whether full-disk access is granted, why it’s important, where to check for that, what the ‘manage plug-in’ button in Mail actually looks like and see if it’s indeed there and the plugin is activated.
Now, you might ask, isn’t a pop-up an over-intrusive way of informing users? I certainly agree, were it not the add-ons screen is likely visited once perhaps twice. It’s not a menu a user goes back to unless some problem arises. If the step to grant disk access is that important and DT cannot check if it was, IMO a user should be ‘taken by the hand’ to check for it manually.
An even better solution in the end might be if the plugin-in somehow provides feedback to DT that it was activated. If it wasn’t, the add-on shouldn’t be greyed out and reported to be installed I think, as the mailbundle might be ‘installed’ in the library, but Mail doesn’t use it necessarily if my observations are correct. But that requires a method for the mailbundle to provide feedback, and I don’t know if that’s possible.