Here's a little adblock list to hide Nitro upsell buttons in Discord! You can still purchase Nitro by temporarily disabling your ad blocker, and Nitro features will still be available with this list enabled. This just keeps the noise down for all the times you aren't buying Nitro.
To add this list to uBlock Origin, open uBlock's settings page and click on the "Filter lists" tab. At the bottom of the page, expand the "Custom" tree and check the "Import..." box. Then, add this URL to the textarea that appears:
https://raw.githubusercontent.com/synthead/discord-adfree/main/discord-adfree.txt
Click the "Apply changes" button on the top-left of the page, and you're done! This URL reflects the latest commit to this list, so your ad blocker will auto-update accordingly.
This list can be used on all the other common ad blockers too, but they have their own settings page that I won't get into detail about. If you don't know about uBlock Origin, you might consider switching to it as it's fast, light, and is GPL-licensed.
Enjoy!