I used to provide compatibility information to add-on developers so they can prepare their add-ons. This happens early (during alpha phase) so people can plan ahead. The bulk of the compatibility discussion takes place shortly after beta 1 is released, at which point developers are encouraged to review their add-ons and make announcements.
As for the messaging from NV Access: according to latest wiki documentation, one release per year is designated backwards incompatible release. This is so developers can assess things such as dependencies and trends and make adjustments. This does result in add-ons needing to be updated, and for this reason, releases such as 2019.3 and 2021.1 include an extensive set of changes for developers in the what's new document. Based on discussions with NV Access and from within the add-ons community, NV Access and add-ons community encourages developers to keep track of changes and respond accordingly.
In case of my own add-ons, I prepare my add-ons during alpha phase. Although there is a risk of needing to adopt to changes faster, I feel at least it gives me a piece of mind, knowing that add-ons are good to go by the time beta 1 ships. This is why all add-ons I'm activley maintaining are compatible, and that's the reason why I sent out a notice about the future of my add-ons and upcoming nVDA releases last night.
I, too, share users' frustrations that your favorite add-ons are not working in NVDA 2021.1. I have advised the add-ons community to make a statement about it, and I'm sure NV Access will at some point to assure users.