Android P could allow blocking of unknown number calls in addition to allowing uses to block calls from private numbers and calls coming from pay phones, which would extend the capabilities of call blocking currently available in Android Oreo devices and devices running on older versions of the Android software. This enhanced call blocking comes with a wide-casting net of possible options so that users would essentially be able to blanket a group of numbers they don't want to receive phone calls from, at least that's the belief at this point based on the data found in a recent commit on the Android Open Source Project website.
Based on that data and if Google does end up adding enhanced call blocking to Android P, users would be able to section off and block calls before they're made from numbers which are either private, unknown or unlisted, from pay phones, or just from numbers which aren't stored in their contact list. Users could end up toggling all of those categories so that calls coming from any one of those groups are blocked before they come in, though users would likely want to use this with caution as sometimes calls come in from people you may know whose number fits into one of those groups.
There's no guarantee that Google will add this into Android P, but it is a possibility. There are also other possible scenarios. Android P will be the developer preview version of Google's upcoming Android OS version and developer previews happen in stages. Google could keep this from the developer preview and simply add it at the very last stage when the software is getting ready to be pushed out to users. That said this is likely a feature Google would also want tested with users and developers, so if it is to be a newly available feature with the next Android version it is likely to show up in one of the preview builds at some point. This would be a big improvement over the call blocking that's available now where you have to individually and manually enter numbers into the blocked list. Though it is worth noting that it seems carriers will have control over whether or not this feature is available, so even if Google adds it to the software then carriers may individually decide to prevent it from being used on their networks.