Civ has been a game where modding is a thing for the last 2-3 civ games. It really changes the game for the better, that was my experience with Civ5 and Civ6, and I believe Civ7 will be the same. There are many subjects where people just don't agree, so the devs will leave those open for mods. Like religion, Anti-Aircraft guns, more units, more civs, etc. are all gonna be possible via modding.
And yeah I am aware of external modding software, I have it myself. I still would prefer Steam workshop though since I could just send a link of my mods to my friends who can subscribe to them at an instant. For now without the Steam workshop, what I do is to zip all the mod folders and send that zip to my friends so that we could play multiplayer.