The question in the title is a clickbait, no one ceases to be a founder of an establishment, institution or company, even if that company collapses. And as @Wera's answer clearly states
A founder is a founder, dead or alive, even 300 years later
The real English language question is whether someone can be described as a “former founder”. And the answer to that question is: Yes, if it part of a predicate compound
Articles by Jeff Atwood
If one of the two founders of Stack Overflow is called Jeff Atwood and chooses to describe himself as co-founder (former), it appears that the reference was meant to be tongue-in-cheek, which is in keeping with the software developer's sense of humor.
One word that best describes how I work: I'm going to assume that an animated GIF counts as one word. Here you go.
Life Hacker Q&A
In a tweet, dated 1 February 2018, Atwood wrote
I have not worked at Stack Overflow in any capacity since 2012, but I occasionally dip my toe into meta.stackoverflow.com and I am so inspired by the way the community collectively carries the vision forward
On his Twitter profile, the former CEO of StackOverflow does not disassociate himself from the company, on the contrary, he describes himself as
“Indoor enthusiast. Co-founder of stackoverflow.com and discourse.org .…”
The Ngram chart below shows, however, that the expression "former founder" is not so rare as one might initially suspect
Citations from Google Books, in no specific order
For a good part of 2007, the former founder-owner-CEO and his (potential) ideas remaining a frequent topic and reference point
Complaint of Federal Savings and Loan Insurance Corporation, as conservator for savings and loan association, alleging fraud on part of former founder and principal shareholder of association pleaded fraud…
There was Alexander D'Arbeloff, the former founder, chairman, and CEO of ARD affiliate Teradyne, who is today a professor at MIT's Sloan School of Management, and the honorary Chairman of the MIT Corporation, the university's governing...
James Gustave "Gus" Speth, former founder and president of the World Resources Institute, a research center on environmental issues, and a former founder and staff attorney at the Natural Resources Defense Council
In fact, the TI's strategy, perfected by Peter Eigen, former founder president of the organization and defector from the World Bank, encouraged the construction of “coalitions” between state, private sector, and civil society…
Dean Leffingwell is a renowned software development methodologist, author, and software team coach. He is the former founder and CEO of Requisite, Inc., makers of RequisitePro, and a former vice president at Rational Software, where he was responsible for the commercialization of RUP.
All of the citations above are perfectly grammatical and make semantic sense. The expression "former founder" is used in compound predicates
The predicate is the part of the sentence that makes a statement about the subject. The predicate usually tells us what the subject is doing or what is happening to the subject.
A compound predicate tells us two (or more) things about the same subject (without repeating the subject).