off the top of my head, i guess canonical avant garde would include people like john cage, terry riley, tony conrad, la monte young, early yoko ono, etc.
would add:
Robert Ashley, Bernard Parmegiani, Alvin Lucier, Ingram Marshall, Tod Dockstader from the Concrète crew, some of the Soviet composers who worked with Tarkovsky and the ANS like Eduard Artemiev, Vyacheslav Ovchinnikov & Oleg Buloshkin, from Germany Wolfgang Dauner, Floh de Cologne & Xhol Caravan but theres so much good gear from this place & time you could go right off the deep end on Krautrock alone via its own thread, Alan Splet/Ann Kroeber, Kevin Ayers, Severed Heads, Jon Hassell, Patrick Cowley odds n sods, Craig Leon up to Weatherall's comedown/mid 90's projects & that delicious Mica Levi soundtrack for Under the Skin
music theory pretentiousness states that as there are no new chords or notes to explore (note the word tone is excluded), "relational music" is the current avant-garde = out of key piano runs immediate cut to people in a public park sample immediate cut to water down a sinkhole......un-listenable bs where the concept takes precedent over the intrigue
the drone thread offers real pleasures usually associated with this field, but out of the names not really listed Andrew Liles is someone who deserves respect for keeping things uneasy, pushing his studio chops and he's one of a handful of producers worth buying on sight. Zoviet France & certain Rapoon gear? Jeff Bridges of all people did a safe-mode/F8 hate to use the word concept lp recently (Sleeping Tapes)
its unhelpful trying to frame things as avant-garde because then you have to define it, its historiography & the fact that a lot of acts, eg: Jim O'Rourke & Jim Haynes as 2 from hundreds of names, can wade into these waters at any given time.....the list is metaphorically endless but proper fun exploring, you could live a few lifetimes & never really engage in it all