I had been under the impression that Office for Mac 2016 and 2019 were the same software versions as their 365 counterparts, receiving the same monthly updates, until they are "frozen" three years after release (like Office for Mac 2016 was at version 16.16.x).
However, ribbon customization is only available for 365 subscribers; it is disabled for Office for Mac 2016 and 2019 users. There is no pre-purchase mention of this that I can find, other than that the 2019 includes "Classic" versions of the apps. The unavailability of Ribbon customization is only mentioned in this thread or the app itself, as far as I can tell.
Is there any list of other features only available for 365 subscribers, so that I can best inform my clients who are deciding between 365 or one-time purchase?