Google's Pixel lineup has historically shied away from dual SIM functionality, but a few new commits in the AOSP code repository, along with an associated comment from a Googler on a particular commit, seem to indicate that the functionality will be coming to some future Pixel devices. To be specific, the commits in question center around getting native system-level support for dual SIM functionality, and one comment on one of those commits outright states that the "2019 Pixel" will support two SIM cards.
A big part of the code is for telling Android whether a device supports dual SIM functionality. According to the above referenced comment, this is important to the Pixel lineup because past devices have lacked dual SIM capabilities, but the feature will be available on at least one of the Pixel devices coming out this year. The issue with that non-specific comment is that there's no way to know if the Googler is talking about the Pixel 4 family, only one variant of that lineup, the Pixel 3 Lite series, or all of the above.
Background: Google's Pixel series, and its progenitor, the Nexus series, has never had any kind of dual SIM support in any region. This is a very popular feature for globetrotters, as one may imagine, or even those who like to keep separate lines for business and personal use.
That being the case, Pixel devices picking up dual SIM functionality is a natural next step for the series. It stands to reason that Google wants the Pixel lineup to be taken seriously not just in its home market or among Android purists, but also with a worldwide audience across multiple mobile use cases. Including dual SIM functionality is a big step in the right direction, in that regard.
It's worth noting that Pixel and Nexus lovers haven't been entirely left out in the cold when it comes to dual SIM functionality, even though it's never been available natively on their devices of choice. To be specific, there are a number of third-party workarounds available. One example is MagicSIM, a system that hooks into one SIM card inserted in your phone, while leaving another dangling on the end of a ribbon cable, to be secured by a case.
Impact: The impact of this move should be obvious; a crowd that has previously written the Pixel line off for not having dual SIM capabilities will now have an opportunity to give the phones serious consideration. This feature may sound niche to some, but being able to switch SIM card lines on a whim can be a pretty big deal. You could use that to have one plan for calling and texting versus another for data, or switch over to your work number during the day, and go to your personal phone after you're done.
The larger impact of a bigger crowd being able to consider and appreciate the Pixel family is that Google will have an easier time moving product, which should be good news for all Pixel fans, whether they care about dual SIM functionality or not. If Google sells more Pixels, quite simply, the company will have more motivation to devote more time and money to development and quality control.
Even so, don't expect the usual glut of early adopter Pixel issues to go away any time soon; that comes with every iteration, and usually consists of problems that only surface at the kind of scale enabled by real-world usage by thousands of buyers.