Twitch does not disclose what methods of spam prevention they implement for obvious reasons.
While technically it is possible to send whispers, there’s no way to be certain if your account and the message you want to send will be caught by whatever anti-spam measures there are.
Because of this, as I’m sure you should be aware if you’ve read other posts is that Whispers is not designed for bot use and so things such as Extensions would fill the need of what most bots want to use whispers for.