Same for me and users of my LiveTraffic plugin in X-Plane. OpenSky used to be very reliable, but since about 3 weeks ago users started reporting timeouts. I'm using the REST API with endpoint states/all?
One or two requests work lighning fast as usual, then timeouts occur.
My plugin works with automatically increasing timeouts, so ultimately the timeout increases to 60 seconds, which seems to be about the wait required, means: One response comes through. That automatically reduces the timeout
I can confirm this behaviour when just sending off REST API queries in a browser where I could time the response when waits occur: Always about 60 seconds.