Your knowledge is now arcane black magic. The reorg probably shuffled away the people who knew that you knew what you knew, you should be proud.

Having left and (briefly) returned to the same team, absolutely this, 100%. People typically avoid touching functionality they don't understand, for (legitimate) fear of breaking it in unknown ways. New features and changes are built up on top of the existing edifice, rather than trying to change the edifice itself. You end up with an unstable tower of layers in which it's more and more difficult to get anything done -- but not impossible, and not suddenly more difficult all at once. It's a slow boil.