Both the Apple Watch Series 5 & 6 have persistent screen functionality. However, if you are using third-party apps such as the Stryd's and lower your wrist (which would make the screen go black with the older watches), the screen will blur and you will only be able to see the time. With Apple's native workout app, you will be able to continue to view metrics all the time, but this is the only app that will allow this behavior.
We really really hope Apple extends the always-on functionality to other apps, but we have no information on when/if that will occur. If Apple ever enables third-party apps such as Stryd's to use the always-on display, we will do so.