To save you the trouble of looking it up since U.S. residents were never presented with the Kindle Paperwhite 3 and 4 labels, the Paperwhites whose software will no longer be updated are those that are not waterproof. Simple dividing line.
https://goodereader.com/blog/kindle/amazon-has-ended-support-for-the-kindle-paperwhite-3/amp
Amazon has ended support for the Kindle Paperwhite 3
Michael Kozlowski 18 hours ago

Amazon has ended support for the Kindle Paperwhite 3 and the e-reader will not receive any future firmware updates. The only exception is if Amazon upgrades their Kindle Whispersync program and Kindle Store with a new version of TLS.
The 7th generation Kindle Paperwhite 3 was one of Amazon’s most successful e-readers ever. It came out in 2015 and was heavily marketed for four years, before it was eventually replaced by the Kindle Paperwhite 4 in late 2018. The last time the PP3 received a firmware update was in 2019, and it is currently running 5.12.2, while all of the other modern Kindles, are running 5.13.1.
Amazon frequently suspends firmware updates on devices that are 4-5 years old, as a way to get people to always upgrade to the latest and greatest, if they want to receive new features, bug fixes and enhancements. Kobo tends to issue firmware updates for most of their older devices, going as far back as the Kobo Mini, that was released in 2012.

Michael Kozlowski is the Editor in Chief of Good e-Reader. He has been writing about audiobooks and e-readers for the past ten years. His articles have been picked up by major and local news sources and websites such as the CBC, CNET, Engadget, Huffington Post and the New York Times.