The bug
Setting the blur slider to 0 outright disables the blur shader (MC-269032). Despite this, it does not display "OFF" like other sliders do when set to 0.
How to fix
Make it say "OFF".
MC-270265
Blur slider does not say "OFF" when set to 0%
Resolved
Fixed
3
Can confirm, affects 1.20.5 pre-release.