Now we have some more information about what these parts are. It appears the Core processors with F suffix will lack integrated graphics. The decision to launch these processors may be linked to Intel's 14nm supply issues.
As you may know, the chip giant's 14nm capacity is fully utilized as too much of Intel's product mix is now on 14nm due to the long delay of the 10nm node. By creating a new line of processors without integrated processors, Intel may be able to supply more processors to clients that need chips for applications that don't require integrated graphics.
Basically, the F-line enables Intel to sell silicon with a defective graphics part, which boosts the effective yield. These parts are unlikely to become widespread.
According to the rumor, we can expect Intel Core i9-9900KF, i7-9700KF, i5-9600KF, i3-9350KF, i5-9400F and i3-8100F processors.