All modules that call a Unix library contain WoW64 thunks to enable calling the 64-bit Unix library from 32-bit PE code. This means that it is possible to
run 32-bit Windows applications on a purely 64-bit Unix installation. This is called the new WoW64 mode, as opposed to the old WoW64 mode where 32-bit applications run inside a 32-bit Unix process.
Old Proton builds probably won't backport this (unless it's completely isolated, idk the code layout of Wine). But are old Proton builds still necessary? Occasionally there's regressions, but are there really any games that require like a 2 year old Proton build?
There are, but it's complicated. Doom (2016) for instance - it doesn't handle the very large Vulkan swap chain that's possible on some modern graphics cards, crashes on start-up. Someone patched Proton around that time so that Doom would start; the patch was later reverted since it broke other games. Other games based off of that engine - couple of Wolfensteins, Doom Eternal - have the problem fixed in the binaries, and so run on up-to-date Proton, but depending on your hardware, only a few specific, old, versions of Proton, will do for Doom.
Regressions get fixed - that's okay. Buggy behaviour which depended on regressions that got fixed - that's a problem.
Does this change run the 32-bit .exe using x86_64 instructions? From the description it just sounds like it allows 64-bit Linux libraries to be used in place of 32-bit ones, but that the Windows layer still operates in native 32-bit mode. This means there is still a need to emulate 32-bit x86 instructions which I don't think box64 can do at this time (x86_32 translates to arm32 with box86, x86_64 translates to arm64 with box64). If box86 could translate x86_32 to arm64 then this might work as Wine would handle the conversion between 32 and 64 bit addressing and argument passing into the libraries but I'm not familiar with the inner workings there.