Video Summery (Spoilers Ahead)
What happened? El killed the monster by creating a portal inside the monster to the Upside down ripping it apart. She was then sucked into the upside down before it closed. El and Hopper met in the upside down on the way to the one Stable portal.
Hop told El she couldn’t go back to Mike or our world because the “Bad Men” would find her and take her. She also blamed herself for the monster hurting her friends. She agreed to pretend to be dead for a bit and lay low. (Hence not being in the 1 month later if the final episode).
Evidence: Mike rebuilt her hideaway in the basement and didn’t seem to broken up about her “death” when recounting the events of the show to Will in the hospital bed. When he looks at the empty El cave I think it’s more of a “I wish you would come back soon” not a your gone forever”. Hop also left Ego’s in the woods.
Conclusion: El is hiding in the upside down or forest until the Government forgets about her.
Speculation: Will has been incubating monster eggs that will attack from the sewers. The only way to stop them will be for them to go to the upside down and get El. She might be hiding out in a Cabin since episode 7 of season 2 is named “The Secret Cabin”.
On Oct 24th Instagram User Milliebobby_brown posted a picture of Millie Bobby Brown with David Harbor (Hop) saying “shhhhh don’t say a word”. https://instagram.com/