The encode plus has been out for about a year and the main difference is that it supports some HomeKit features that the original encode does not. And that it’s intended to be available in either Wi-Fi or thread, with much better battery life on the thread model.
The thread version was released, but without matter support.
There have been several devices intended for use with HomeKit that had thread, but not matter, including the original versions of the eve devices, some of the nanoleaf devices, some Belkin devices, and a few more.
After the Schlage lock was already released with thread support, The matter spec was finalized, and Schlage decided they would not provide matter support for that lock, but instead create a new matter over thread lock. And eventually they stopped advertising the encore plus as being thread capable, probably to avoid confusion with matter.
The following is an article from back in 2022, so I am only including it so you can see the details of the announcement where Schlage said they were no longer intending to update the encode plus to support matter. Just remember when reading it that it’s an old article so a lot of the other comments are now out of date. For example, as of the time of this posting, Amazon’s support for matter has been greatly expanded from what it was back in 2022.