via make.wordpress.org => original post link
As of WordPress 6.1, the send_headers hook has been moved to slightly later in the WordPress loading routine (ticket). Historically, all the is_ functions (like is_singular) wouldn’t work when you were determining which headers to send. With this change, moving `send_headers` to after WordPress parses the query, those functions now work properly.
People can now have more control over things like:
Managing caching behavior