My memory is fuzzy as well, but I recall some sparks here https://lists.gnu.org/archive/html/emacs-devel/2013-12/msg00552.html I also vaguely recall that he didn't like the vibe of a part of the Emacs community and this was a factor in his ultimate decision to quit Emacs. It's a pity - for me he was a really great Elisp hacker and package maintainer. 

On Mon, Feb 19, 2024, at 9:33 PM, Dmitry Gutov wrote:
> On 19/02/2024 20:08, Bozhidar Batsov wrote:
> > And the contributors to Flycheck and Flymake were always pretty 
> > different - part of the reason Sebastien (the original author of 
> > Flycheck) quit Emacs were some attacks he was getting on emacs-devel.
> 
> Speaking of attacks, I don't remember anything significant.
> 
> Sebastian's decision to shut the project down was quite a surprise at 
> the time.
> 
>