On Apple devices Reeder 5, NetNewsWire and Feedbin all use the Mercury parser API (hosted by Feedbin) to extract the full article text. I believe Fiery Feeds also includes this functionality (with a subscription) although I am uncertain of the process used. Lire is yet another reader that includes this functionality by default.

Of those above I recommend Reeder 5 if you don’t care about web accessibility and Feedbin if you do.

If you're on a Mac or iOS, NetNewsWire can fetch and extract the whole article for an individual item, or you can set it to automatically do it for an entire feed: https://netnewswire.com/help/mac/5.1/en/reader-view