The Permissions-Policy: interest-cohort=() header has been removed in Backdrop versions 1.26.
This header had been added to Backdrop websites by default in version 1.19.3. The addition was in response to the implementation of Federated Learning of Cohorts (FLoC), which was a way to gather user data without cookies, regardless of whether a website loads any tracking scripts.
The recent removal was due to widespread blocking by websites, and the fact that no browser other than Google Chrome had implemented FloC. Support for this header was dropped in January 2022 and it now causes a warning to appear in the Console on Google Chrome and other Chromium based browsers:
The previous version of this change notice is included below for reference.
By default, a new Permissions-Policy: interest-cohort=() header is now present on Backdrop websites. This header is added in response to the implementation of Federated Learning of Cohorts (FLoC), which is a way to gather user data without cookies, regardless of whether a website loads any tracking scripts. As of April 2021, Google's Chrome browser is the only major web browser to implement FLoC.
To disable this header and allow FLoC for the website, a new Backdrop setting, block_interest_cohort, can be set to FALSE in your settings.php:
$config['system.core']['block_interest_cohort'] = FALSE;