While all other criteria have changed to use underscore to seperate words, killedByTeam still uses camelCase as the only one. teamkill and KilledByTeam were actually both left completely untouched, which might be unintentional.
MC-122932
killedByTeam objective criteria still uses camelCase
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Stats are being rewritten, but those objectives are not stats or related to that. Feature request, not a bug.