According to Eurogamer, the OK emote was taken off amid its controversy as a hate symbol by white supremacists. It was also added into the database of hate symbol by the Anti-Defamation League (ADL) in September 2019, citing its fingers shaping a "W" and "P" for white power.
"In 2017, the “okay” hand gesture acquired a new and different significance thanks to a hoax by members of the website 4chan to falsely promote the gesture as a hate symbol," the ADL wrote on their description, adding it was then widely adopted online as a trolling method.
The emote in Modern Warfare went along with an overall sign of approval - something the ADL described as deeply ingrained from its overwhelming everyday use.
A number of players online claimed the emote was removed without a notice by Infinity Ward and Activision, while both respective developer and publisher has yet to respond on the decision.
Earlier in June 2020, both companies pledged to combat hate by adding more safety measures. This would include stronger ID scanners to root out racist nametags and easier reporting features for Multiplayer, Co-op and Warzone. Their last major announcement was also posted to players on the menu in response to the killing of 46-year-old George Floyd on May 25, 2020.