According to a report by XDA Developers, recent commits to Android Open Source Project's (AOSP) Gerrit, Google "will be adding property values in the framework and Telephony service to tell if a device has hardware support for multiSIM functionality". This eSIM isn't widely used, though; Sprint in the United States uses it, but few other carriers do. While technically the Pixel 3 does offer dual-SIM support, the SIMs could not be used at the same time.
For those unfamiliar with dual SIM support, it generally works in one of three ways.
The second, Dual SIM, Dual Standby (DSDS), allows a phone to receive calls and texts from the secondary SIM as long as the phone isn't actively using the primary SIM for the same goal.
Pixel 4's rumored DSDS mode, which sounds like it will run on the same SIM/eSIM configuration as the previous Pixels, should work more like Apple's iPhone XS: When both SIMs are inserted, activated and standing by, a call or text can come through on either. The most common is Dual SIM Dual Standby, wherein both SIMs are registered on the network at once, and both can receive calls / SMS, but generally only if the other SIM isn't already being used for that goal.
Before then we might well get treated to the Pixel 3 Lite - Google is rumored to be readying a mid-range version of its 2018 flagship phone, built with slightly cheaper materials and a less powerful chipset on board. When one SIM is being used, a call or text coming in to the other SIM can not be received. However, this is rare as it requires the device to have two radios. Interestingly, the same commit stated that the dual SIM capability can be enabled on Pixel 3 devices, even though it's only for dogfooding. What is likely, however, is that when the improved dual SIM feature is made available, it will be present on the next Android Q OS update, along with the new Google Pixel.