Dave Rupert shares a similar frustration to mine, a lack of accessible public APIs, in much the same context - building personal apps or side projects. Recently, I had the idea of graphing the crowdiness levels across Barcelona's metro network at different times but there doesn't seem to be any API or dataset to help provide the much needed data. As an aside, the mobility-barcelona repository by GitHub user maariionaa-code has some good links. Back on topic, Dave provides some of his own examples along with commentary on the fragility of relying on external APIs to begin with - looking at you Twitter and Reddit.
Coincidentally, Terence Eden recently followed up on his post from two years ago on APIs that didn't require authentication. It doesn't look good today, but there's still some useful ones out there.
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