I'm an intermediate level user of reddit. I use the "reddit is fun" app on android. It took me a long time to discover that the i in a circle at the top of the screen will bring up the sidebar content.

I have a suggestion, though I'm not sure it is a good one: If the sidebar contains a certain number of links, or length of text, and the user hasn't opened it on their mobile app yet, then the app can highlight the button with flashing or a color to recommend that a user reads it.

I find that there are many subreddits where if you start reading posts without the sidebar you are really missing out.