I hadn’t heard of this before either. A quick search (miraculously, Google still sometimes works for me) indicates two separate phenomena: one is that HP did intend to insert printed ads when you use some of their automated print options (https://www.computerworld.com/article/2519039/hp-partners-wi...), and the other is that some HP printers are “web-enabled” in the sense that emails to a certain printer email address will automatically be printed, and the default access settings allow spam ads which then get automatically printed (https://h30434.www3.hp.com/t5/Mobile-Printing-Cloud-Printing...)
Turns out, it was an official practice by HP.
I'll never use any of their products again.