I've seen it quite often that people refer to someone as a "former founder" or "former co-founder," but that's not really possible since a founder is always a founder. Once you found something, even if you leave the project that you founded, you are still a founder. You can't be a person who was at one time the founder of a project, but is no longer the founder. What's the proper way to refer to this phenomenon?
I would suggest founder emeritus.
The Compact Oxford English Dictionary defines emeritus as an adjective meaning:
(of the former holder of an office, especially a university professor) having retired but allowed to retain their title as an honour
From what I have seen, this is either handled with "Founder (no longer with the company)", or something like "Founder and former CEO of ...". I do not recall seeing any single-word alternatives to these.
Nice question :)