The easiest way that I have found to look for F availability on CX regional flights is on Google Flights. Select Class = First, then enter HKG as point of origin, then "MNL,BKK,TPE,SIN" as destination (can have up to 5 points of origin and 5 destinations separated by a comma and a space). Select Stops = Non-stop, and Airline = Cathay. Then click on the little 31 symbol to the left of the departure date and scroll through the months. Doing this is, it is clear there is no F on CX flights to any of the 4 destinations mentioned after March 24th.
It looks like from April onwards only HND will have CX regional flights with F.