Having used an early kindle and a recent kindle, they are incredibly similar. One of the main innovations of the new models appears to be adverts you have to pay to get rid of.
My kindle from 2012 used to have ads you needed to pay for to get rid of. It was sold as separate product with or without ads at a time. I had one with ads.
I keep it offline in airplane mode permanently from 2016 and haven't seen a single ad in a long long time.
Actually, the old Kindle had physical buttons, which I find more ergonomic when reading in bed
Also gradually phasing out support of formats like mobi, in such subtle ways that if you open a mobi file you cannot go back to the library, but have to cold-reboot your device...
My current kindle is my third one, and is the last. I will never ever pay for a kindle to Amazon, due to its user hostility.
Oh, and also you cannot move ebooks between accounts, even not with a lot of friction, eg. support tickets, which would be a fair way to game piracy and unwanted lending, which was some inconvinience for me in a situation. Not a huge monetary loss for me, rather a reminder that when you pay to Amazon (or Valve, or any other contemporary DRM-burdened vendor) you are only leasing...