In short: Many users who have updated to Android Pie are now saying that their head unit is unable to connect to their phone and open up Android Auto. This issue does not appear to be affecting all head units, or even vehicles with Android Auto built-in. It appears to be affecting mostly Pioneer after-market head units, and some Android smartphones (though there are only a handful that have Android Pie right now). Google has addressed the issue, and is working with Pioneer to push out updates to fix the issue. On its Product Forums, there is a table showing which firmware will fix the issue for each individual Pioneer head unit. So that users are able to get the update onto their head unit and get Android Auto working once again.
Background: Android Auto appears to break pretty frequently when new versions of Android come out. It's unfortunate that Google's teams don't talk to each other, especially the Android and Android Auto teams – not to mention the fact that Android Pie was in beta for nearly six months before it rolled out. But Google does at least get things fixed, though some can take longer than others. This particular issue appears to be an issue at Pioneer though, and not at Android Auto specifically. Many users have tried using different USB cables to get things working, and that's not the issue here.
Impact: This is not affecting everyone, and as mentioned it's really only affecting Pioneer head units and some Android smartphones running on Pie. So that is a pretty small group of users. But Google isn't leaving them out and is pushing out updates to fix the connectivity here, which is definitely a good thing. It may be a bit of a pain to update the head unit, but it's definitely worth doing if you do use Android Auto all of the time.