Here's a 2019 Google blog post about Google's user studies to redesign their Material Design text field. They ran three separate studies testing over 140 combinations of 7 text field characteristics with 600 participants. The end result is better but they basically just discovered that users couldn't identify text fields that had only subtle visual affordances (like a light gray underline instead of a black rectangle). The exercise reminded me of the Google designer story about testing "63 shades of blue links".

https://medium.com/google-design/the-evolution-of-material-d...