Ad-supported or not has always been a choice for as long as Amazon has offered ad-supported kindles. I remember making this choice about 10 years ago, and I made this choice about a month ago when I bought my most recent Kindle.
If you bought it used or from a reseller, I don't know what to tell you, but Amazon gave you a choice if you bought it from them.
As a source, here is an Engadget article from the time when this option was introduced: https://www.engadget.com/2011-04-11-amazon-releases-ad-suppo...
You can clearly see that Amazon offered the "with Special Offers" version alongside the versions without "Special Offers". The ad-supported version was discounted by $25 back then. Nowadays, it appears to be a $20 difference on the Paperwhite.
Not having to hack the device you supposedly own to avoid having ads as your screensaver? My Kindle is at the bottom of my closet for that reason. I haven't bought another physical e-reader, but if I do, it won't be a Kindle. I didn't want to go through the process of finding up-to-date instructions, waiting for a hack that would install on my hardware and software version, crossing my fingers to do the install, at the end of which I wouldn't even know if it was a victory for me or a victory for them that I was still using their product. So I stopped using it and won't buy another one.