Technically, the pull-request still provides a legacy option (although it's not documented). Users who upgrade to the fixed version can still maintain the original format with this custom date format:

%d{"yyyy-MM-dd HH:mm:ss,SSS"}

That's essentially what users are currently doing to get the correct ISO8601 format. Instead of adding "ISO8601_xxxx", I'd propose new formatting options. For example, add an "old" option to enable the legacy incorrect format:

%d{ISO8601,,old}

In any case, I'll poll the mailing lists as suggested.

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