File system settings page gives "'temporary' is not a supported key" and does not save (11.x)

Problem/Motivation

In Drupal 11.1.1 saving the File system settings page (/admin/config/media/file-system) gives the form error message:

'temporary' is not a supported key.

and the form is not saved.

I looked into the HTML markup and there's no form element with the name "temporary".
The only form element existing is the select: with name temporary_maximum_age.

I also looked into settings.php and there's also no such key.

Any ideas what may cause this?

Same issue on all Drupal 11 projects, while Drupal 10 projects work fine. So I guess it's a Drupal 11 issue?

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet