It’s not like he actually has feelings for Deku. He’s just making sure no one can hurt the little fucker. He still maintained that sweet, innocent naivety from high school, despite everything that had happened to him. Katsuki had seen people try to take advantage of that and he’d been the one to comfort the damned nerd when his heart got broken again and again.
So why not just cut the issue off at the root–leave no space in their lives for someone to fuck with Deku again.
No one has gotten close to the nerd since Katsuki took it up to be his personal hero. Good, because Izuku doesn’t need anyone except for Katsuki.
Katsuki and Izuku go on an away mission together and it tests Katsuki and his "not feelings" for the nerd.
Katsuki saw a crying little girl at the airport with long green curly hair, red carmine eyes and a freckled heart shape face. Katsuki was enamored by the little girl's beauty and stopped his tracks. Bewildered.