In short: The latest Android security update includes a fix for the original Google Pixel and Pixel XL in relation to fast charging, and once affected devices are updated it's understood the issue will have been addressed. This problem has never been associated with the newer Pixel 2 or Pixel 2 XL, and not all 2016 Pixel and Pixel XL phones are understood to be affected. Therefore this patch will only be relevant to owners who have encountered the issue.
Background: The first wave of reports on this issue started to come through in early August after 2016 Pixel and Pixel XL phone owners started to notice a fast charging issue had arose following the devices being upgraded to Android 9 Pie. As the user reports began to build, Google actively acknowledged the issue and confirmed a fix would be coming. Since then there has been little mention of the issue from Google with users having to continue to use their devices with the problem in effect. This now seems to have been remedied with the latest security patch, at least according to the official literature accompanying the update's release.
Impact: With this being an issue that only affects the 2016 models of each Pixel phone, and not all models, and with the issue seemingly more prevalent when a third-party charger lacking USB Power Delivery support is used, this will be a patch that only applies to a select number of Pixel phone owners. However, for those owners this will be a welcomed fix as the inability to make use of "rapid charging" is one that not only will prove to be frustrating, but has real-world implications by slowing down the time it takes for either of the phones to reach a full charge again. At the time, Google never really explained why the issue has occurred in the first place beyond the suggestion the Android 9 Pie upgrade had caused an issue with non-Power Delivery chargers. While the update bulletin does now consider the issue fixed, there's one again no firm explanation on why the issue surfaced in the first place, or how it's actually been patched.