The recent update allowed players to get to mons to 39.5 - it was a Niantic fix that only went half way.
However, it appears that Niantic have remove the current update from rollout, as it is very buggy on many devices.
I would expect that when the next update comes out that we are able to get mons to 40...but that is an assumption not a guarantee.
TL/DR The new update only allows you to get to 39.5
EDIT
https://www.reddit.com/r/TheSilphRoad/comments/7b53fd/0811_rollout_delayed_at_all/?st=j9oudq28&sh=4b093044
https://www.reddit.com/r/TheSilphRoad/comments/7ayzr5/has_the_new_update_been_withdrawn/?st=j9ouddqt&sh=7ed37554