You can, actually. See "Mac-only license" in their pricing [0]. Just as before, you get one year of updates.
True, when Sketch started switching to a subscription model, they've buried the link to renew the license very deep so it was really hard to find, but I guess they've came back to their senses and it is now available right on the pricing page.
This must be very recent. I was looking at their marketing collateral in May, and it was simply no longer an option at all to buy a Mac-only license. You could only renew at the time.
True, when Sketch started switching to a subscription model, they've buried the link to renew the license very deep so it was really hard to find, but I guess they've came back to their senses and it is now available right on the pricing page.
[0]: https://www.sketch.com/pricing/