As of PHP 7.3, a warning will be thrown when continue is used within a switch without a numeric argument.
Ref: php/php-src@04e3523

The feature to use continue within a switch is not deprecated nor slated for removal.
Also see: https://wiki.php.net/rfc/continue_on_switch_deprecation

So code using continue in a switch is still compatible with PHP 7.3, they will just get a warning thrown.

Should a sniff be written for this ?

Asking because, again, this could be considered a grey area.

Note: I'm in favour of sniffing this as I've already seen Travis builds running PHPUnit break on this (as PHPUnit exits with an error code because of the warning).