There is no plugin at /plugins/fields/itemlist/itemlist.php in the core of Joomla. It must be an extension you have installed.

That's interesting since it is just a Joomla content custom field that is causing the issue. I will make an attempt to see what other extension might own this, but I am already surprised.

@bascherz Check the sources here and you will see it is not in: https://github.com/joomla/joomla-cms/tree/4.2-dev/plugins/fields

I have already looked at the 4.2.5 distribution and confirmed it is not part of that. The extension manager reports that the author is "Joomla! Project" and it is version 3.9.0 of the Fields-Itemlist Site Plugin. Is this residue from before I migrated this site to Joomla! 4? I disabled the plugin and it doesn't appear to have made any difference (editing the field still works).

When you use my link and then switch the branch from 4.2-dev to 3.10-dev, you will see it is also not part of Joomla 3.10. Sometimes extension devs just copy the copyright comments from the core.

Well, wherever it came from, it doesn't appear to be needed. It also was singled-out with a core-extensions filter in extension manager. Gone. Thanks guys!

Closing as not a core issue.