Looks like another attack on them. Old EULA is present again.
Most likely, this software is dead and author does not intend to publish it. There is also active plagiarized CMS with similar key generation algorithm (minor changes made).
Author's email address is pikusov@gmail.com if you wish to contact him.
Fortunately, software like Opencart is much better than Simpla CMS.
Looks like Simpla is WTFPL Version 2 at the moment (note version number was not changed). It is archived in Wayback Machine but link is currently broken (may work in future).
Old EULA is still present on website, but you can bypass it by using this link.
Not sure whether it is actually available under this license or website is attacked once again, but license checks are removed from this code.
English version (2.3.7) is not [yet] available under WTFPL but IIUC it is outdated version with Russian strings replaced with English ones (something is still in Russian).
Is Simpla free or no?