I'd wager that most owners of Pixel phones use the fingerprint sensors on the back to unlock their phones, but there are circumstances in which using the backup PIN, pattern, or password is necessary. Many Pixel owners, myself included, have noticed that it's become rather difficult to swipe the lockscreen up on Android 8.1 Oreo. Google is aware of the problem and is looking into it.
The most prominent issue is having to swipe all the way to the top of the screen to reveal the PIN/pattern/password input, but that seems to only be because it's the most-used action. Users on Google Product Forums, Google's Issue Tracker, and reddit are also reporting problems with swiping notifications away, as well as swiping the notification bar down. This has been a problem for some time now, but Orrin, a community manager, has just replied to a Google Product Forums post saying that the company is looking into it.
Hopefully, the company tracks the issue down quickly, as having to swipe repeatedly to unlock your phone is a bit of an annoyance. In the meantime, if you commented on a Google Product Forums post with this issue, keep an eye on your mailbox for an email from Orrin and a request for a bug report.