I just checked out your latest changes concerning package version information because I have to implement it for Lilith when the next version of Logback is released.

At the moment StackTraceElementProxy does not have a getter for the contained StackTraceElement. It would be really helpful for me if I could access it. Otherwise I'd have to parse it from steAsString which I'd really like to prevent.

I'd suggest to implement both a getter and a setter for the StackTraceElement.
steAsString would need to be set to null if the STE is changed.
Additionally, I'd suggest to make steAsString transient since it can be restored after deserialization.