There's a huge difference between not selling F and changing the schedule, downgrading all flights to aircraft w/o F.
At this point it's just SQ being careful trying to avoid ending up in a situation where they have to rebook people because they changed the aircraft last minute.
Considering SQ's fleet, it is not unlikely that once borders open again and demand picks up, quite a few destinations will be served with a 4-class aircraft.
However, it's impossible to predict which ones that'll be, so they're trying to be safe rather than sorry