Not even kidding; cassette. Before you get all up in arms about the releases, remember how nostalgic this is for some, how cool looking it is for all, and how different of an idea it is now that vinyl is a popular thing.
I'm just going to let Prosthetic do the talking here about what releases they'll be doing and where you can get them, since they've put it so nicely for me already:
Prosthetic Records will soon offer select releases as limited-edition audio cassettes. The first batch of analog titles include Blacklands, the Prosthetic debut by female-fronted doom rockers Castle; Ride the Void, the sophomore album from metal shredders Holy Grail; and the new release from Boston grind/thrash outfit Ramming Speed, Doomed To Destroy, Destined to Die (out June 25). Additionally, the label will offer cassettes of some top-selling releases from its catalog, beginning with Lamb of God’s New American Gospel, Trap Them’s Darker Handcraft and Skeletonwitch’s Beyond the Permafrost. Pre-orders are available now via the Prosthetic webshop.
All of my yes. It's nice to see that at the very least, they're thinking outside the box and trying some different stuff. On a fan level, it's cool because you're getting a different way to physically own your music, you're getting a little bit of a variation on the artwork, and you're getting to hear music like you haven't heard it before. Sure, it might be worse quality than what you're used to depending on your system, but that's what metal was. You're hearing it like it was when it started out and you're owning it like it was. I'm a metal history nerd too, so there's that.