Yes, iOS 13 will be able to automatically silence calls from unknown numbers and also automatically send them to voicemail.
Here is what Apple writes about the new "Silence unknown callers" feature on its detailed list of new iOS 13 functionalities:
You will be able to choose whether you want to use this feature or not, and this might not be a great fit for people getting a lot of real calls from unknown numbers, but it sounds like something quite useful for a big majority of the population. Currently, there are a few third-party alternatives, but this one will gather information these three sources within your phone and should be able to get a pretty good grasp of your contacts. Of course, important calls from an unknown number will still be accessible in the voice mail.
This is not quite as advanced as what Google offers with its Screen Call feature that will do a live transcription of an incoming call, so you get an even better idea of the intents of the caller, but it is still an important step that will protect your from those pesky spammers.