It's occurred to me... most of the time posters have been talking about a 'copyright holder' in the singular, with the assumption that Commodore was the copyright holder of Workbench/AmigaOS and it got somewhat murky after 1994.
But I'm not sure Commodore WERE the only copyright holder. For certain aspects of Workbench they were a licensee... for example, Commodore's license for the Narrator library expired in 1991 - which is why it wasn't included in later versions of Workbench. Cloanto had to license that independently, which they did for the first time in 2002. I think there were other components of Workbench for which Commodore was a licensee, e.g. ARexx and Memacs?
So to get the original Workbench disks released into the public domain, you'd probably need to get a whole list of copyright holders to contact, not just the entity which has inherited all of Commodore's copyrights. I get the impression Cloanto have done this.
Otherwise, even if you did work out who has inherited Commodore's copyrights, you'd only legally be able to distribute specially modified disks (or ADF images) which removed any component for which Commodore had only been a licensee, and not a copyright holder.