Most likely, Samsung will not update the Galaxy Note 5/S6 Edge Plus beyond Android 7.0 Nougat but there is an easy way to upgrade and install Android 8.1 Oreo + root by installing a custom ROM. This is the Lineage OS 15.1 custom ROM for the Galaxy Note 5 and S6 Edge Plus, which works quite well even for a beta version.
For those of you looking to try Android 6.0 Marshmallow based on TouchWiz with root for T-Mobile Galaxy Note 5 SM-N920T, there is a preview version you can try out right now.
This is an early edition of what the final firmware will look like and everything works but the battery seems to drain faster than usual. Of course, I do advise you to wait until the official version is out but if you cannot wait or simply want to try it out, here’s how to do it.