Synchronous review is still possible today! One of my earliest managers taught me that if a "standard" code review goes back and forth more than once, it's almost always better to just hash it out in person (or on a Zoom call, when at least one person is remote) and then go back and post a comment summarizing what consensus was reached. To use a contorted technical analogy, asynchronous text communication can be lossier in terms of what information it's able to successfully encode than verbal, and the throughput is lower, so sometimes it's worth it to pay the synchronization overhead when you need to exchange a lot of information.