Are ideas becoming harder to find?

I don't know if there's a way to answer this question definitively. I believe ideas and innovation roughly follow a sine curve of progress. As a society, we are perhaps in the second half of the downward sloped-part as of this writing.

Interestingly, I found this website by Patrick Collison and Michael Nielsen - https://scientificreturns.org ^a which has a well-researched list of content.

Technology takes a while to enter the realms where it will have the most impact.

Other Readings

Peer-reviewed papers are getting increasingly boring by Daniel Lemire

Noah Smith takes a softer stance on the idea of science-is-getting-harder in the article Answering the Techno-Pessimists, Part 4: Science Slowdown?. The author distinguishes between "slowing down" and "very difficult to sustain", and that's where the nuance is.