Flycheck requires Emacs 26 these days, so I think it makes sense for all extensions to do the same. I'd also suggest depending on a newer Flycheck version, as 0.18 is ancient. Probably it'd be best to depend on the latest stable version (currently 34) - Emacs package repos don't keep multiple versions of the same package and always serve the latest version.
Flycheck requires Emacs 26 these days, so I think it makes sense for all extensions to do the same. I'd also suggest depending on a newer Flycheck version, as 0.18 is ancient. Probably it'd be best to depend on the latest stable version (currently 34) - Emacs package repos don't keep multiple versions of the same package and always serve the latest version.