The Brief: Typing "f" into a chat is a way for people to "pay their respects" to something or someone.

DEEP DIVE
The term originated after a feature in the 2014 release of Call of Duty asked players to press F on their keyboard or X on their controller to “pay respects” to virtual fallen soldiers.
Since then, the term and action have been co-opted as a way for people to playfully “pay their respects” to any number of things in Twitch chats and other comment threads.
Stan Lee has passed, F's in the chat. pic.twitter.com/KOPsHnazov
— Will Sperling | MoonBaseKappa (@moonbasekappa) November 12, 2018
after 1 entire year of hiding in bushes my dad caught a W yesterday can i get an F in the chat pic.twitter.com/J6wRwvVTSl
— soulja boy still owes me $10 (@BillRatchet) November 4, 2018
chris is getting a 50% in science, can I get a F in the chat to pay some respects
— Aizuru (@AizuruLOL) November 7, 2018