Upon loading a world from 1.16 pre-release 2 to pre-release 3, Control option 'Swap Item With Offhand' was reset to default key 'f' from previous change I made 'key.mouse.4', which is 1 of 2 buttons on the side of my mouse.

Note if relevant: No other controls were changed. I had set my 'Sneak' option to 'key.mouse.5', which remained unchanged.