We're nearly certainly going to get Android Q arriving with the Pixel 4, ready to take advantage of that dual-SIM functionality. This eSIM isn't widely used, though; Sprint in the United States uses it, but few other carriers do. The AOSP Gerrit is an online collaboration tool where developers who can share code changes - called commits - and merge them into Android's source code. Crucially, the commit says it can do this "even if [the device has] two or more SIM cards".
Not much is known about the device for now (a few modest hardware upgrades are probably inevitable), but courtesy of a report from XDA Developers, we now know that it could ship with improved support for dual SIM cards.
For those unfamiliar with dual SIM support, it generally works in one of three ways. That's called Dual SIM, Single Standby (DSSS), and it means that while these phones are technically dual SIM phones, they can't connect to two carriers at once. DSDS is what's used by most dual-SIM Android phones and the newest iPhones; here, the second SIM can receive calls and texts if the main SIM isn't being used at that moment for the same objective. So, the minute that you switch over to the eSIM, you're no longer able to make or receive phone calls from the number assigned to the physical SIM in your smartphone. Ideally, phones would have Dual SIM Dual Active (DSDA) capabilities but those require two wireless radios instead of one. Specifically, the comment cites the implementation as necessary to differentiate the 2018 and 2019 Pixel - in other words, the Pixel 3 and Pixel 4. XDA is guessing that the Pixel 4 will support Dual SIM, Dual Standby (DSDS), which is certainly a logical conclusion.
For those who want to use dual SIM functionality to juggle a business and personal number, that's a pretty serious setback.