A fatal error ends the request, and before PHP 8.5 its message usually only showed where it happened. The new setting adds a backtrace, the chain of function calls that led to the error, which helps developers find the cause of a crash more quickly.
PHP's advisory links it to CVE-2026-109619. On PHP 8.5, the confirmed fatal backtrace path is one way to reach the stale global state bug, and the advisory says to set fatal_error_backtraces=0 administratively and restart the workers. It adds that this blocks only that confirmed path, so it is not a complete fix.
It applies to PHP 8.5 only, since earlier branches do not have the setting. Hiding the message with error_reporting, display_errors or the @ operator does not help, because the problem is not in what is printed.
Treat it as a stop-gap. Update to PHP 8.5.12 or later on that branch, then decide whether you want backtraces on again for debugging.