llll The last stable release should still be available for download because even an older GrapheneOS device is more secure than the stock google android that the phone comes with.
I can understand that perspective, and I can also understand why the GrapheneOS developers might not want the project name to be associated with devices that can't be supported, or to have those devices calling up the GrapheneOS update server.
An interested party could clone the final release for a platform, change the branding and the updater web site, and make images available, perhaps under a name like "ExpiredOS".
Please note that I do not speak for the GrapheneOS project.