In SysLogAppender stacktrace lines are treated diffrently from the main message.

Basically, no filtering is done, and the suffixPattern is not used on the stackTrace lines, so there's no way of controlling these lines.
The appender hardcodes a tab in front of each line, that's all

I've modified SysLogAppender to support a stacktraceSuffixPattern parameter.
This parameter allows you to configure filtering for stacktrace lines, just as you do for the main message with the suffixPattern parameter.

You can use this to modify (or remove) stacktracelines sent to syslog.

The code is here:
https://github.com/ingebrigt/logback

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