I've been watching boat leaderboard for a few months and I see missing farm there. Combined with other observations, here what's I'm thinking:

Some maintenance break (if not all of them) will somehow "reset" some leaderboard and few places in the game
- Farm needing help tab : the farm won't be listed there until player logs in again (I know coz' I set up a permanent flag on my other farms, they never show up after a maintenance break)
- Boat leader board: players are normally listed once they reached a minimum of points needed, but aftermaintenance break, they only get back on the board once they earned new boat points (help wth crate or send a full boat)
- town : farm with townie list seems to disappear but I haven't watched enough to know the conditions for them to appear again (either new townie release or visit town or logs in)
- derby leaderboard : I heard there are funny stuff going on there too but not playing derby, I cannot check. Either the team earns nez derby points or players logs in or whatever else.

So, back to player inactivity : I'm using the boar leaderboard to check who is recently active and fill boat crate. Let's say that maintenance break of this type occurs at least once a month. If a player has been inactive from more than 1 month, he/she is very likely to not appear on the boat leaderboard between 2 maintenance breaks. It means player doesn't fill any boat crate, even during boat event.