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It has been asked the longest mate that includes draw rules is, and it is known what the longest tablebase mate is. However, what is the longest tablebase mate, i.e. 7 pieces or less, that accounts for the 50-move rule?

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Unfortunately looking for the longest mates on Kyrill Kryukov and checking with Syzygy won't give you the answer. The mate lengths ignoring the 50 move rule don't correspond with the mate lengths when the rule is in force. For example this position with a ply count of 0 under the 50 move rule is mate in 112, but ignoring the 50 move rule it's mate in 91.

[Title "White to move, ply count 0"]
[FEN "6k1/p7/8/8/7N/7K/2N5/8 w - - 0 1"]

Indeed the longest mate in this endgame with the 50 move rule in effect is 128 moves from the following position, which is mate in only 85 if the rule is ignored. But that is with a ply count of 4 or 5. (The longest mate in this endgame without the "draw" rules is only 115).

[Title "Black to move, ply count 4 or 5 "]
[FEN "8/8/5N2/p7/8/k1K5/8/1N6 b - - 0 1"]
[startflipped ""]

In fact none of the currently available tablebases will give you an answer. What you need are DTM50 tablebases which I believe are currently in progress.

See here for a full explanation: http://galen.metapath.org/egtb50/

In general if the ply count exceeds 3 the mates could get even longer owing to the necessity of avoiding triple repetitions (though that wouldn't apply in the above case). So you may not have an answer even with the DTM50 tablebases because the DTM50 tablebases wouldn't take this into account. (They would play perfectly accurately from ply count 0 positions, i.e. if used by an engine from the time the endgame appeared.)

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    Bother. So the answer is "no answer yet". At least I upvoted. (Community, does Martin "deserve" the bounty under the usual SE rules? I want to play fair here, which might also be NOT handing it over!)
    – Hauke Reddmann
    Commented Nov 2, 2021 at 9:09
  • Wow, until you mentioned it, I hadn't ever considered the role of avoiding triple repetitions in this context. That almost seems like cheating, to lengthen the mate by forcing the winner to avoid a long list of specific positions that have already arisen twice before in the game!
    – Timothy Chow
    Commented Mar 19, 2023 at 0:27

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