I am unable to download Teams from App Store because it says it requires iPhone OS X version 14.0 or up while I am unable to install this version on iPhone 6. My iPhone is up to date and it has OSX 12.5.5.
Support is limited to the two most recent major versions of iOS. See here: iOS version history - Wikipedia. This would be OS 14/15. In this case you would need a new device with a modern OS. I myself just moved from IPhone 6 with OSX 12.x to an IPhone SE and am now currently on OSX 15.2.
Even if you could install it, it's not supported and wouldn't receive updates. You would probably have issues with other Microsoft apps such as Lists and Office.