Is it permissible to replace the "exit confirmation" pop-up window in forums with "Kaffarat al-Majlis" (expiation for the assembly), and would that be considered an innovation (bid'ah) and collective remembrance (dhikr)?

There is no impediment to writing the supplication for the expiation of the مجلس (gathering) — "Glorified art You, O Allah, and with Your praise, I bear witness that there is no god but You, I seek Your forgiveness and repent to You" — in the exit window of an online forum, because it is a gathering where people meet, and transgressions may occur therein. This supplication expiates what takes place in the gathering, and if recited after a good deed, it serves to affirm it until the Day of Judgment. This supplication is prescribed when departing from online gatherings and at the end of articles, and writing it for remembrance is permissible.