Board Game Arena

That spot was held for months by a player 550+ ELO? The player still holds 550+ ELO but is no longer on the leaderboard?

"All-time" means "played in the last 2 months" to BGA. If you don't play, you don't appear in the leaderboard, but you don't lose elo, play once and you'll appear again.

Can you point to this change? It used to be 6 months?

As far as I know it has always (or for several years) been the same, two months, six, or whatever.