Could you elaborate why you think it would help to have this on the mixer?
There is plenty of space to add the master snowflake in the mixer and it wouldn't be hard and it wouldn't break any other feature or cause any problems so I guess I'd ask you...why not?
Freezing/unfreezing all tracks is probably a pretty common thing and it is typical in good UI these days when you have a stack of options that can all accept the same command that you allow the user to set/unset them all at once and not have to recall which menu that feature is buried on. If you are working in the mixer it makes sense to keep your focus there while you are using it.
Why do we have an option on the mixer to mute all tracks? Same exact logic applies there! We could remove the mixer entirely and just put all the options in menu panels but obviously having the mixer improves workflow.
This is an example of what Pipeline refers to when he laments how slowly things change here compared to products like RapidComposer. In that product if someone suggests a timesaving feature that has no downside and is relatively easy to implement it will appear in a day or two in the next build!
But here, even such a simple and easy little request has to go through forum scrutiny and even then no one knows if PG is even listening until a year later when the new version comes out and whoever requested the simple feature has completely forgotten about it, found a work-around and moved on.
I have mostly lost interest in this feature request already!