Chris Evans may be leaving his role as Captain America, but he is already working on the Avengers 4 movie that is in production at this time. So, he would be leaving it after Avengers 4 and Avengers 5 has not been started. Therefore, it can’t be said for certain that Chris Evans will no longer be Captain America in the Avengers series. Only time will tell, but it would not be a good move for Chris Evans to stop his role of Captain America after doing so many movies.
However, there have been movies that Chris Evans has made and the time needed to spend on the movie conflicted with the time he needed to spend on Captain America. There may be another movie that is conflicting.