I couldn't find a Thread about it and I had a lot of doubts, if using Discord, link account mechanism and Discord Server Stats are allowed. Does anyone have any idea whether or not it's allowed?

I would think any Discord Linking plugin would NOT be allowed due to the fact as you need to use chat commands to complete it. Discord Server Stats should be allowed since it is similar data you can get using a steam server query or rcon. This assumes you want to keep your server in community and not modded.