This is a PSA for all players that have been on 10.12.6 (like me) to avoid certain bugs in Blizzard games that crop up in later versions - as of today, July 1st, the Battle.net App requires 10.13.6 to even launch. If you are not on that OS you cannot launch it and thus are unable to even patch or install the game. This goes for every macOS game Blizzard has that uses the Battle.net App for patching.
It’s time to upgrade, whether we want to or not.
you are complaining about a 5 year old OS losing support. if they havent fixed those “certian bugs” in 5 years, maybe its a good sign to move on?
There are also certain features I rely on that Apple removed in 10.13 and beyond (the FTP server is a prime one). There are some situations where software and other needs dictate when you can easily update. This is mainly to let others know that this is now a hard requirement, not a soft requirement as it has been for a while.
Oh, and for D3 players, the salt here lies in that this particular game still uses OpenGL, which is beyond buggy and in some cases almost unplayable on modern macOS versions because Apple hasn’t updated OpenGL in eleven years and the new graphics drivers they release for their OSes have barely functioning OpenGL support.
Edit: FWIW, four hours later, I’m updated to 10.13.6, which is the latest OS I can use with a 1080 Ti. Now I get to go pay for a license for Transmit since Apple so thoroughly screwed over Android users by removing the FTP server.
I am waiting patiently for the new 14" and 16" MacBook Pros in the fall! My current 2012 15" MacBook Pro- running 10.15.7 still runs Diablo Pretty good! Phew. Can’t wait! (I may end up with new new 24" iMac if I don’t like the new MacBook Pro configs)
About time you do. You certainly got several years out of your setup. 
The so called M1X or M2 at the earliest is Nov/Dec (best guess), but as I said in another post the 24" iMac runs the game quite easily in Rosetta 2 not getting too hot. The even more powerful laptop Macs should be a delight to use, with less heat, and way better battery performance.