Redecker
Redecker's whole tradition predates plastic brushes, and it shows: handles are untreated wood (often beechwood), bristles are natural fiber - tampico agave, horsehair, palmyra, coir - and the dish brushes use a stainless-steel clamp so the worn head can be swapped and the handle kept for years. The only non-plant part is that metal spring or ferrule, which is inert and recyclable. For dish and bottle brushes, Redecker is about as close to fully plastic-free as the category gets.
How clear are their specs?
Names the exact bristle fiber (tampico, horsehair, palmyra) and the wood on each brush, and sells matching replacement heads.