When upgrading iphone 4s to OS 9, iTunes said it could no longer support my phone. Need to upgrade iTunes. Then when I tried, my MacBook Pro doesn't have the OS for the new itunes. Why didn't my computer tell me before istarted?

Good question, I have no answer...

This is common practice for Apple. Always ask here or ask mr. google before you upgrade any Apple machine because Apple will never tell you.

There appears to be a lack of clarity over this question. iOS9 seems to require iTunes 12.3 but others have said they get it to work with lower iTunes versions as long as they do the iOS upgrade online and not with iTunes. Apple's iOS9 online manual says iOS9 works with 10.6.8.

One thing is for certain. Apple does not use this forum to respond to this kind of question so do not anticipate a response here other than lots of other "me too" type of posts from people in a similar position. If Apple does see a need for clarity it will likely either be a change in the online manual or something else "just happening". It is also vaguely possible some enterprising user may find a way to get things to work but I haven't seen such a post yet.

You can be a squeaky-wheel here: https://www.apple.com/feedback/ Maybe Apple will get out the old oil can and add a couple drops.