sysupgraded from 19.07.8 to 21.02.0, keeping configuration. Only the 2.4GHz radio is enabled on this router.
Under 19.02.x this was correctly reflected in the LED configuration: 2.4 was on and flashing when there was activity, and the 5GHz LED was off.
Under 21.02.0 this has reversed, so the 5GHz LED is now on/flashing when there's activity on the 2.4GHz radio, and the 2.4 LED is off.
Edit: Okay... After a reboot, the radio LEDs are back to the old, correct behavior.
I noticed this problem on the first reboot after luci prompted for the wireless and network configuration migrations, so this might have been a migration artifact.
Leaving this here in case anyone else bumps into it.