Building an online service and API is a difficult business. Once you've actually spent the time putting it together, writing up good documentation and delivering something reliable and stable, you potentially have to support it for a long time. But not forever. In Dropbox's case, it pulled the plug this week on its original API, v1, leaving tardy developers — and their users — with potentially non-functional apps.
As Dropbox states, developers have had fair warning regarding the transition:
A little less than a year ago we announced June 28th, 2017 as the cutoff day for API v1. Developers have asked us for more time to finish updating their apps, so we have extended the API v1 cutoff date until September 28th, 2017.
A quick check of a calendar will show September 28 has been and gone and with it, the endpoints for v1. That means the API isn't just deprecated — any apps that rely on those endpoints will simply stop working (well, their Dropbox functionality at the very least).
For most people, this won't be a problem, but there will be a not insignificant number of apps out there that rely on the old API that haven't been updated.
If you're affected, best to hit up the creators directly and ask for a patch. Otherwise, the hunt for alternatives begins.