How to get rid of this stubborn note popping up on right bottom corner everytime I open Forum page ?...

Nothing you can so to get rid of it.  Just keep closing out, or ignore.

There is a close button, just use that, while it can be annoying I would imagine it’s there to help newcomers to the forum, whether it’s helpful I suppose that’s the debate.

It seems that you visit the DJI Forum regularly... Me, too.

Yep it is always there when I get in , a nice reminder if you feel a need to rant or worse.