How can I turn off these marketing notifications? I don’t want to disable all notifications since I use reminders sometimes, but I don’t want to see this spam in my Notification Center either.
There should actually be only one notification, just confirmed this on 3 computers using 3 different macOS versions. Which macOS version do you use?
I’m on the latest version of Big Sur.
But, how can I turn these off?
Only by disabling notifications for DEVONthink in System Preferences > Notifications.
It would nice to have an option for this.
Notifications are a service nicely provided by Apple, so the “option” is nicely available at Apple’s MacOSX: Apple Icon → System Preferences → Notifications 
That’s a global setting where you can turn off all notifications for DEVONthink. As I wrote above, I use some notifications, like reminders. I just want to disable marketing ones like new releases.