Most UA's that care about font-size actually have 9px set as a minimum, but I hold the rule that 0.875REM should be the smallest you allow being set. The smallest default size one tends to encounter in the wild is 1REM == 12px (old sun workstations), as it is generally used to scale upwards not down. Windows doesn't even include the option to go smaller.

But really if the user wants to go that small, that's on them, not you as the developer. So long as you stick ot 0.875rem which is 14/16ths, you are blameless if it ends up below "accessible" sizes. It's now on the user, not you as the developer.