Geralt probably ought to text or call before he stops in, but he doesn't care all that much for these devices. He's lived over a century without any kind of instant communication that wasn't face to face and he prefers it that way. It's... easier.

Makes more sense.

It also means he can stand at Yennefer's door as long as he needs to without her expecting him. Not long, in the balance, but there's a few long minutes. Knocking means accepting that she didn't keep her word to let him know how things went. He's allowed a moment to be disappointed by the failure.

He does knock though, twice, solid. Before he can stand in the hall long enough for it to feel awkward.