I think it makes sense to have custom field enabled at the level of Project. However, it would be very useful to be able to select which custom field to display on a per task basis. Right now, my team has a dozen custom fields in on project but most tasks only need 1-2 of them. So there's a bit of visual clutter happening.
It seems I recall a little eyeball icon when I was in Task > Manage Custom Fields, that allowed one to hide/show select custom fields. I believe I use this on 10/5/18. Dev experiment perhaps?