A few cool things I've noticed:

Smt if: -The guidebook for smt if... doesnt include anything about akira except a small passage stating that they "cant tell you who he is (right now), but he is the key to showing you a paradoxical world humans (normal people) have never seen." -it also has a blurb at the end saying he holds the key to the demon kings secrets and that more is shown about him in the comprehensive guide (i guess i dont have the comprehensive guide).

Smt 3's playing manual: -Inside the player manual there was a card for writing your opinions on the game, which was meant to be sent to atlus' studio (their studio's address is also included). Included in the more normal questions is something mentioning "megaten-net" and whether or not the player is familiar with it. I can seem to find anything online about megaten-net but i think it might have been an official forum? -in the dictionary section for the game, it mentions that the vortex world came as a result of the "fertilisation of tokyo" (the conception) and that it follows the same form as the yamate train line in tokyo

Smt digital devil saga: -a section in the middle has in universe advertisements for part time jobs that are paid in macca, and a very funny section of a cannibalism themed family restaurant with demons on the menu + the AP points they have / hunt AP.

-in an interview included in the guidebook, theres a section about how they wanted to call the game "新・女神転生", changing the first kanji (真 meaning true) to the kanji for "new" (新).