He goes back and forth on that, sort of like Downey did after Iron Man 3. Russos recently stated Evans isn't done yet, so who knows.
...the entire time I was in Aquaman I was sitting there thinking, god I'd love to see Namor on the big screen. I hope they do, do that. Although I know Universal still has his distribution rights, Disney does have the production rights, like Hulk, so they could use him, just probably not in a standalone solo title. I'd much rather he be in a film with Cap than to have them stick him in the Black Panther franchise as a villain so I hope Things Fall Apart's theory is correct.