Will SIM-free iPhone from Apple lock to first carrier activated? Suppose I buy an iPhone 7 directly from Apple, and I select the SIM-free version.  Will this lock to the first carrier it is activated with?  Or is it unlocked for life so I'll be able to switch carriers freely without restrictions?

Searching online, I found some reports that claim past SIM-free iPhones might lock to the first carrier they're activated with (even if bought via Apple), but the sources don't seem fully credible to me.  Going through the purchase process on apple.com, Apple describes the phone as SIM-free but never calls it unlocked and doesn't seem to have any definition of what SIM-free means. That makes me wonder.
According to Apple a SIM-free iPhone purchased directly via Apple should be unlocked. So, it is pretty unlikely that you get locked afterwards.

Furthermore you should be able to use every SIM worldwide*.

*considering there are no restrictions in the chosen country like e.g. in Turkey.

Edit: This answer is based on the situation in the U.S. only. Other countries/markets may vary.