Newer Office 365 client builds are installing it by default. Worse, they didn't move away from the per-user installation. Instead they just took the brain-damaged route of force-installing it inside each user's profile when they logon. I think that's a real dick move, but, then again, I sometimes think I'm the last admin in the world who gives a damn about efficiency.
Again it depends. I downloaded an installer only yesterday that did not have it, even when it was part of the subscription. It is also not being pushed as an office update to my users yet. But then again I haven't checked if that is because WSUS is holding it back
If you take a blank W10 machine and do a 365 fat client install on it you get the ‘Teams machine wide installer’ installed on it, which then hooks the teams installer in on a per login basis. Your workplace must have endpoint management in place that somehow prevents this.
They will overtake Slack easily since it is a free addition to most 365 plans