This would have the effect of opting a user in to whatever the default is (as opposed to keeping the blank value and inheriting from the default) at that moment in time, which would make it so that if the default ever changed the user would not see that change. Is that the intention here?

Or maybe more specifically - is the lack of visibility into what the default is the issue? Could we display that somewhere if so?

I think it is fine to lock the time zone, just as the locale and theme are locked.

If the default time zone is changed in the future for whatever reason, it is unlikely that all the users want to have their effective time zone changed as well. You usually only want to change your time zone when you are (semi-)permanently moving to a different country/state.

Sorry, to clarify a little bit -- if the TZ selection defaulted to the default TZ setting, it would wind up being set for any user who submitted the form, regardless of whether they intended to do so or not (ie, they came to the form to change their language pref or something unrelated to TZ). If in the future the default changed, they'd be on the old default from having submitted the form. This might be totally fine! -- just wanted to highlight.

I get your point :-)

I think it's fine. I don't think users want/need the option to “follow the instance default time zone, even when it changes”.

In fact, we might even populate User#time_zone already at account creation, just like we do with the theme, so the time zone doesn't change unexpectedly at some later time. What do you think of that?

I don't have a strong opinion on the "set a default at creation time" vs "leave blank and always inherit current default" aspect.

I wonder if this change was just changing the selected/default label (but not submitting a value) if it'd be more mergeable, and we could leave that defaults/inherit question for later.