yes.
just to be clear, trying to access an undefined variable was also “wrong” in PHP 7, it generated an error message at the “NOTICE” level. In PHP 8 it now generates a “WARNING”; also the default error reporting level in PHP 8 has been set to “E_ALL”, while, before, by default it wouldn’t show "NOTICE"s.
Anyway, I guess you can choose between always setting a variable before using it (The Right Way™), or just provide a fallback value via the null coalescing operator when accessing the variable:
<?php
if($os ?? null) {
//stuff
}
?>
<?= $os ?? null ?>