Hello @Yra !

Welcome back!

>How advisable is it to use UNA as an SEO platform?

Could you plz share the necessary functionality? But by default, UNA has enough SEO tools for work.

>What happens if I delete the UNA key, will the site stop working?

No, you will just lose the possibility to use the Apps Market and upgrade the core and modules via the Studio. But the site will work.