Feldman's second string quartet (https://youtu.be/QPMUHVza-KA) is very long, has lots of repetition, with occasional pulsating rhythms/drone notes. I quite like it when I'm in the mood. There's a lot in it, but it's so slowly developed so slowly moment to moment it has characteristics of minimalism.
Well sure, most of Feldman's work could be described as "minimal" but saying it is part of the body of work that is typically referred to as "minimalism" is a little disingenuous, in my mind.
But maybe it just makes it more obvious that these categories aren't particularly useful. I think it's interesting to talk about those early works in the 70s and compare them to minimalist art, like Judd's or Serra's sculptures or paintings by Sol Lewitt, Stella, etc., but it's just a component of a much broader conversation that needs to be based in listening first.
It's interesting to me that Glass' early music didn't really click for me until I had stood inside of one of Richard Serra's Bands sculptures at LACMA. It gave me a different mental (and metal, haha) framework to engage with the music through. That's probably why the Berio/Feldman comparison was so surprising to me in the first place, because I feel like the music from those composers both demand different kinds of engagement to find them enjoyable that Glass' early music, both as a listener and a performer.
Indeed, much of Feldman's late period fits this description. I would also recommend Piano and String Quartet [0] [1] from Feldman as an extreme example of his interests in minimalism.
This short article covers some of his work including the second quartet from this perspective in slightly more detail: https://www.nytimes.com/2002/02/17/arts/music-where-minimal-...