Interesting. Waveshare makes this recommendation across their product line, and from what I can glean from google, their displays really do develop permanent artifacts if they're not refreshed. I've never seen another epaper company say this, and anecdotally I've seen ereaders that held the same image for months or years work perfectly fine, even with no power for refreshes. I don't know why there would be a difference.
Happily, e-ink displays don't suffer from burn-in.