> "To keep the trend honest we computed it on a fixed panel of 594 companies that appear in our data every year, 385k reports in total, so the decline you see is the same companies getting stingier."

Isn't there a selection bias here? The average tech company that has stayed in business since 2015 presumably grew quite a bit and can now be choosier.

39% seems like a lot

Source is unclear. Self-reported by candidates? Huge bias then, since people may or may not want to say whether they got / didn't get an offer, and whether they accepted / rejected an offer, and this split may vary by company and by year very significantly.