To chime in..
I am in complete agreement with cptomes.
Forcing users to upgrade to a later SW release in order to use a peripheral is quite frankly a little silly.
If there is a solid, sound reason why the drivers/functionality cannot be backported, i am all ears.
I noted that elsewhere in this thread there was a statement that it was clearly (and i do emphasise this word) stated on the box that 10.11 is a requirement.
It is indeed noted, but i must say that grey on white in very small lettering is hardly clear.
A further addition i would add is that in my case i purchased the mouse out of frustration , missing the left right scrolling ability of the solution provided by my IT department, the only available mouse in the 'mac area' of the store was MM2 , so it's not like i was given a choice or warned of the 10.11 requirement.
Like others here , i am not able at this point to upgrade to 10.11 (if i wish to have a supported system for work).
Looking forward very much to a solution (other than putting myself in an unsupportable configuration) from Apple on this.