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;
Introduced in branch: 
1.19.x
Introduced in version: 
1.19.3
Related Github Issues: 
https://github.com/backdrop/backdrop-issues/issues/5103