weird decision, many addons are not installable from the settings, and instead of improving, knowing the problem they just drop it!

Hi Waleed
I had a slightly more positive response than yours from Odoo , who knows ... ;-)
It appears that this module no longer exists in Odoo 16 and there is no standard way to integrate GitHub with the Discuss app.
However, we have submitted feedback to our R&D team for this module. Please note that I cannot guarantee exactly if or when any changes will be made. Typically, our team uses feedback from our customers to improve the next versions of our software (most likely Odoo 18).