If you intend to purchase an item from the merchant anyway, why would you pass on 20% off?
I sign up for newsletters to get a discount then immediately unsubscribe. If merchants are going to offer a discount for me to input my email, copy the code they email me, and GMail unsubscribe why would I turn that down?
So … ops normal?
If you get more than "insignificantly little" spam in your inbox, you are using the wrong mail provider.
My email address is on every spam list under the sun. I get 600 spam messages per day[0], but only a few per week hit my inbox.
[0] It was 600/day before I made a small change to my mail configuration. Now it's only about 50/day which is few enough that, every month or so, I actually check for false positives. I occasionally see a low-value marketing list message that isn't technically spam in the sense of being entirely unsolicited, but content-wise it's not differentiable. Zero legitimate personal messages. I can live with this.
Most discounts I run into seem to be based on incredibly inflated pricess to begin with. If a shop offers me a 20% discount on something it is often cheaper to buy it somewhere else.