where you can find a cheaper key/game of Sekiro Shadow of Die Twice GOTY?

It does get discounted, down to -50% every month or two as seen here. Wait another couple of weeks and it will be on sale again.

It's 50% off every few months. Also it's a From Software game. They are a lot more reluctant with sales/discounts than the average publisher.

Wrong forum, suggest you move this to Help perhaps?
It has been 50% off SEVERAL times - https://isthereanydeal.com/game/sekiroshadowsdietwice/history/
Use ITAD to check for legit sellers like Humble, Fanatical, etc.
If you want key reseller then go to Google, SG isn't the place to ask about those shady sites.

You need to use some Snap Seed in order to see the discount. They're hard to come by, but until you do that, you won't be able to see it.