Looks like there are more features in this update that are likely part of the vague "other general enhancements" in the Release Notes, where there are multiple new options for adjusting the text in a book. I'm sure this is remotely activated post-install, and I also guess that it likely only works on newer Kindle Store books or KFX. This feature may be why the 10th gen Kindles got cut off, and they are likely only going to get security updates instead of feature updates.
https://old.reddit.com/r/kindle/comm...e_update_5183/