logback is widely used, and not just in bespoke systems where the developers read all the release notes.

There are people using software products that use logback, and sometimes the users customise the logging in external files. A non-backwards compatible change would either potentially break those logfiles, or make the product developers recicent to upgrade.

There have been some mistakes with backwards compatibility in the past, but it is something a stable mature library like logback should get right.

Let's look at the pros and cons.

Pros of changing the ISO8601 preset:

• arguably more correct, even though the current definition is not wrong according to the standard, and it trivially worked around by a custom log format

Cons of changing the ISO8601 preset:

• breaks backwards compatibility as a principle
• will break some users logging configs
• sets a precident for allowing backwards in-compatible changes in the future

So it seems there is no benefit in making a change, only problems.