This isn't just a problem for startups (though for practical financial reasons it tends to occur on a bigger scale there), it is basically a huge problem for the entire web app ecosystem in general. In the desktop-centric past there were apps I used that were killed off but I continued using for years afterward because the existing code didn't suddenly stop working when it wasn't being supported anymore. The web gives you no safety net here, once the app is killed the servers are likely to be brought offline within a matter of days/weeks and then you're usually SOL.
Feels bad man.