Having completed a full review of all illegality proof sketches, I found 3 more mistakes. That brings the final counts to 538 legal and 381 illegal, with an estimate of (4.48 +- 0.37) x 10^44.
There is still a chance that one of the 381 illegality proof sketches in
https://github.com/tromp/ChessPositionR ... %3Aillegal
is overlooking something, but I consider it small enough that I'll happily offer a $64 bounty for a proof game for any of the 381.
There are also bigger bounties of $128 and $256 for software bugs, see the README at https://github.com/tromp/ChessPositionRanking
I'll be writing a paper about my results in the months to come.
Happy bounty hunting!