Earlier quoted context omitted.
A ton of ISPs use deep packet inspection for various kinds of filtering (and other shenanigans). When they get it wrong it manifests to the user as certain websites or access patterns being inaccessible and the ISPs customer support agreeing that you should have access and being able to do fuck all to fix it. A VPN in the middle usually solves the issue.
Wait, I think an ISP cannot inspect the content of packets that are encrypted, say, with HTTPs. In order to inspect TLS encrypted packets you need access to the end-device, controlling the end-router is not sufficient since you would not have access to the device certificates. If you can prove that an ISP can inspect packets, it would be major news.
DPI does not require any decryption of payload. Even cheap consumer devices can perform DPI on encrypted traffic. ISPs absolutely use DPI as a part of standard practice, and have been for decades. It is a basic network traffic management tool.