I am having trouble in selecting Landsat bands in Sentinel Playground. I used the former (free) version of this app in tas a tool to visualize gypsum outcrops by using landsat bands 7-4-3. However, with the new App colours do not show the same way (I presume this is because Sentinel bands do not correspond to the same numbers as those in Landsat). I know Sentinel hub allows using landsat images as well as Sentinel ones, but I do not know how to specifically select landsat bands in the playground App. Any help will be appreciated.
I am not sure if I completely understand your problem. The Sentinel Playground is still free to use (here).
Your assumption that the bands don’t match is correct. You can find a short overview about the Sentinel bands here and for the Landsat bands here.
Coming back to your main question:
In Sentinel Hub Playground you can configure your own bands using the custom option (1st entry) in the side panel on the left side. There you just drag&drop the bands you want to use into the 3 different channels (R, G, B).
If you want to switch between satellites called datasets (e.g. from Sentinel to Landsat) you can do this using the small satellite icon in the top right corner.