So what's the modern day shenanigans this time around? I assume they haven't abandoned it.
It's downscaled a lot. There is literally no modern day level with another character. It's more handled like AC: Liberation, where you play 'a game' by Abstergo. But your game is hacked by modern-day Assassin's who give you some objectives (you have to find a certain character for them who Arno should have met, and try to figure out where his remains are).
Only real modern intrusion are the Rifts, like the one you saw with the Nazi Eiffel Tower. It's Abstergo who notices your game is hacked and try to kick you of the server, so you have to dig into the code or something which brings you to buggy versions of Paris, where different time zones connect. There are some sidequests around this, that are part of the modern-day storyline (you have to free assassin's who got stuck in the code)
I'm really glad they dumped the modern stuff. I hated those levels in every AC game to date (except maybe AC2)