Another relevant dimension is that the forum (and Groups) are the most targeted to EAs, so they will be most sensitive to fluctuations in the size of the EA community, whereas 80k will be the least sensitive, and Events will be somewhere in-between.
Given this and the sharp decline in applications to events, it seems like the issue is really a decrease in the size of, or enthusiasm in the EA community, rather than anything specific to the forum.
Another relevant dimension is that the forum (and Groups) are the most targeted to EAs, so they will be most sensitive to fluctuations in the size of the EA community, whereas 80k will be the least sensitive, and Events will be somewhere in-between.
Given this and the sharp decline in applications to events, it seems like the issue is really a decrease in the size of, or enthusiasm in the EA community, rather than anything specific to the forum.