Will Phoenix Point be open to the modding community? In my opinion opening a game to the modding community can only make it better.
I’m pretty sure Julian confirmed they want mod support during one of the interviews/ama, but it will be something that will come after launch.
I hope so. There are over 4,100 XCOM2 mods out there, so it is a HUGE community of creators and subscribers.
This question has been asked a few times, but I think the most succinct official answer comes from the Reddit FAQ:
Q: Will we be able to create mod for Phoenix Point?
JG: Modding support will be added shortly after release.
Answered by Julian Gollop himself; can’t get more official than that, right? 
Thanks. That is all I needed to know to know I will be preordering on the 1st.