I left biglaw in 2014, at which time my firm had barely transitioned from Windows XP. This was because we used certain Word plugins/macros for formatting briefs, and they had not yet been updated to run on newer versions of Windows. As a result, we were all on XP way past its prime.
From talking with lawyer friends, I got the sense my experience was not uncommon.
Unless using Windows XP was somehow making them less money or costing more billable hours, why would they switch to Windows 7 or 10? I wouldn't even think about switching until an EOL notice went out. If it ain't broke...
I hear this a lot, but like others are commenting, I also hear a lot about Windows XP boxes with word 03 setup w/ the macros they need running to this very day. I'd love to see something other than anecdata.
I submit that law firms are actually far more progressive about technology than the tech world assumes, but are also a lot better at sniffing out bullshit software that doesn't provide a real-world benefit.