Did the developer have any special agreement or contract with Twitter that the API would always be available? If not it sounds like an unacceptable professional risk to base your livelihood on the whims of another entity.
Musk famously does not care about contracts. Note that the only reason he ended up buying Twitter is that Deleware's corporate courts really don't fuck around and were likely going to force him to buy it pronto despite his attempts to wriggle out. But he'll happily screw over lesser people, as he's doing with various vendors, like Twitter's landlords: https://www.businessinsider.com/twitter-landlord-sues-compan...
Explain to me how a small software vendor, even with the best contract in the world, is going to force Musk to turn back on their access in a timely enough fashion that they'll still have a business left? And be able to do it for a modest enough amount of money that the ISV will still be turning a profit?
The point was that when you build on top of someone else's platform, [1], then you are beholden to their whims. This is a substantial business risk, which has been happening since forever.
I sympethize with Tweetbot et al, but this day has been inevitable for some time. They are not the first to suffer this fate in tech, and they won't be the last.
[1] I'll add, without paying - ie not being an actual "customer".
I agree that building on other people's platforms is dangerous, but that's true regardless of the contract you have, because contracts only matter to the extent that you can afford to enforce them.
And they had access to a less limited API (request-wise) to make their app work better without constant rate limiting nags.
“Hmm… it is actually the people that have created value for the site by driving engagement that are the true idiots”