Android 10 stopped receiving security updates in February...
Android 11 is a year older... so it probably still has a few months to go. Months. On a new device. And it seems unlikely they're going to upgrade between android versions if they're running a 3 year old OS on release.
Then again, do you need security updates on your ereader? Are you really going to log in to your email, bank account, or whatever other service on a screen that does 15FPS (and is one of the best eink displays at that)?
I mean, what for? Sure, synchronizing books, yeah. I do that, then turn the wifi off. It's just draining battery for no reason. Perhaps synchronizing your notes to the cloud if you use the ereader as a tablet, that I could see for heavy note takers.