Regarding OS X calling it macOS wouldn't be any better since it includes counts of users from both. It'd probably make more sense to just call the group "Mac" or "macOS/OS X" but it's too late to change the name of the category since paid users had been exporting the data into tools expecting "OS X" and it's a silly thing to break that for. Similarly splitting the two into separate categories or changing the display name vs data name is more problematic than using the old name. The OS version breakdown for macs started in 2017, after the name change, and uses the macOS name for the overall category as well as for the newer operating systems in that category so they are definitely aware and keeping up with the name changes where it makes sense to.

If you change the graph settings to include tablets it'll give an in-graph note that they weren't tracking tablets and consoles as different from desktops until the start of August 2012. It'd probably be good to include that note in the desktop view as well though. For iOS that means iPads (launched April 2010) were bucketed as desktop browsing sessions rather than tablet until that point. While it's safe to assume all of those should be relabeled tablet their policy on data changes is after 45 days they will not go back and modify/correct data.