From my perspective, the ratio of new shooters going to F-Class is still far higher. Local participation here still generally favors F-Class over sling. But, with respect to local matches, I think you are seeing F-Class guys "aging out". Meaning, it takes a lot of time and effort to travel to large matches. Most guys can probably only manage 2 or 3 a year (speaking from my own experience). There's almost zero recognition (i.e., incentive) for F-Class shooters in local matches - for example, our own WA State Mid-Range championship hands out a perpetual trophy, plaques, etc... for the sling shooters. In F-Class, only the top F-Open shooter even got a mention.
I bring this up not as a knock per se, but as a segue into my point - which is, once you start traveling and going to the time and effort to work up a load and barrel capable of competing at national quality matches, local mid-range matches (which I think might make up the bulk of matches for F-Class shooters) are mostly useful for load development. And of those, why not go to the ones that actually recognize you as a shooter?
So yeah, I think you see the more experienced F-Class shooters show up to fewer local matches as they cherry-pick the ones where they are:
A. competing against other shooters at their level and not just discouraging newer shooters
B. being recognized, even just by mention, for accomplishing something (winning or placing)
C. able to fit in with the time demands for working up to traveling to national quality matches
I think you see periods where a lot of new shooters jump into F-Class. They're super active at local matches for a couple years and then start to taper off as they build to larger matches and traveling and all the demands that go with that. I think if you were to graph it, it would look a lot like a series of peaks and valleys (java garbage collection anyone?) over time.
Personally, I think sling shooting is a lot more demanding physically and requires far more muscle memory and so requires more consistent time at local matches and practice to build and maintain.