TL;DR: Currently sale links 404 immediately(?) after a coupon sale ends, which may be confusing for the user. They should continue to go to ended sale page, or redirect to game page.
Steps to repro:
0) Make a sale, pick a fancy coupon name.
1) Spam coupon link all over social media and stuff.
2) Sale ends.
3) Both coupon link (<dev>.itch.io/<couponname>) and the sale link (itch.io/s/<id>) now 404. Note that the sale link still works for the developer, but the coupon link doesn't.
4) Everyone is confused, chaos and destruction ensues.
So it seems weird to me not to let people follow ended sale links:
* People would know there indeed was a sale, instead of having followed an invalid link, copied it wrong, or whatever else weird.
* Sale descriptions may contain useful stuff, even if the sale has ended (e.g. I included some fancy statistics in the description of a recent sale.)
* The developer can still see it, so it's stored anyway.