26.0.1 Notes field unreadable. White text on grey background. why?

This is Apple’s new “liquid glass” design. It cannot be changed or turned off. All you can do is try to reduce it a little by going to System Settings->Accessibility->Display->turn on Reduce Transparency.

You can express your opinion directly to Apple by submitting feedback.

Feedback - macOS - Apple

But for what it's wroth my Notes app on macOS 26.0.1 is still black text on a white background.

In the Notes app, tap the eclipse (3 dots in upper right corner of your screen), tap use dark background.