They've had that "this API is deprecated" message for at least 5 years or so. It scared me off from building features on top of the API but they do seem to be keeping it working nonetheless.
It would be awesome and we at ControlShift would use a supported API if there was a for example a REST API for creating and configuring fundraising pages that did not have scarry warnings on it.
There are a lot of use cases that are just not possible via webhooks for making ActBlue a part of the action experience in other products. I've been working on the "buy beers at conferences" approach to product prioritization with their engineering team for a few years pre-pandemic to try and make something happen. ;-)
Nathan