The Brief: "All the feels" means "many emotions."

DEEP DIVE
This idiomatic phrase indicates an experience of mixed emotions. People having all the feels might feel excited, depressed, or heart-warmed–maybe all at the same time. Context is key in understanding what exact emotions the user is feeling.
Get ready for all the feels. The #Oscars is LIVE, Sunday at 8e|5p on ABC! pic.twitter.com/vl4b8nbdNX
— ABC (@ABCNetwork) February 18, 2019
i felt that.
i felt all the feels.
— β (@_niiixxxx) February 15, 2019
This fan meeting CT3 is giving us all the feels. π© pic.twitter.com/R1e5xAa02M
— Los Angeles Dodgers (@Dodgers) February 16, 2019