PHP 8 no longer silently ignores fatal errors. Background processing of profile pictures in our self-hosted instance was failing on system\Images\Gd.php line 224, which throws a fatal error on imagecolorsforindex if the computed transparency index of a GIF is outside the color range of the image. Fortunately, someone else document a fix at https://github.com/processwire/processwire-issues/issues/1299, which worked (after substituting variable names):
// Suggested by https://github.com/processwire/processwire-issues/issues/1299
$transcol = $transindex != -1 ? @imagecolorsforindex($this->image, ($transindex < imagecolorstotal($this->image) ? $transindex : $transindex - 1)) : 0;
// $transcol = @imagecolorsforindex( $this->image, $transindex );
