I thought I had detected a decline in activity in the ABF some time before COVID.
Perhaps this reflects my own personal values more than the reality of the members there, but my take is that about 4 years ago there began a steady, unrelenting increase in threads that were essentially narcissistic. The forum became inundated with threads like "My personal practice log", "My progress in piano", etc., etc. it really took on the stench of a bunch of very lonely people crying out for human attention, but under the pretense of wanting to share or discuss meaningful piano information.
It all seemed very manipulative, and I am very sensitive to being manipulated. So, my own appearances there have steadily declined over that time period. I have also noticed that many of the accomplished pianists whose contributions were very, very valuable to us sincere beginners, also disappeared over that same course of time.
It is sad on many levels. The utility of that subforum has almost completely disappeared. And, it is sad that so many have lost the ability to relate to humans in the flesh, and now turn to the internet to try to satisfy those unmet personal needs.