https://www.phoronix.com/scan.php?page=news_item&px=Arch-Linux-LTO-Proposed (https://www.phoronix.com/scan.php?page=news_item&px=Arch-Linux-LTO-Proposed)
Older ie roughly pre 2008 CPU's may no longer function after x86-64-v2 is adopted. Kind of like dropping 32 bit support but for older 64 bit machines. Useful to know about, to make plans if you have one of these.
I was reading some comments on the Yay AUR page about LTO which is how I came across this, some packages including Yay don't build with LTO. It might soon be the default as several other distros are using it, to make smaller faster binaries.
But that info is sort of outdated, what they now intend to do is create an entirely new set of x86_64-v3 repo packages alongside the basic x86_64 ones, the same way there used to be 32 bit and 64 bit versions:
"Provide a x86-64-v3 microarchitecture level port"
https://gitlab.archlinux.org/archlinux/rfcs/-/commit/b039e0ddeeed0f08bf4df510ea78a6c884439eeb (https://gitlab.archlinux.org/archlinux/rfcs/-/commit/b039e0ddeeed0f08bf4df510ea78a6c884439eeb)
Which is kind of handy as I just found my late C2D 2009-2011 mobile CPU is only V1 when I checked ;D
$ /lib/ld-linux-x86-64.so.2 --help
Subdirectories of glibc-hwcaps directories, in priority order:
x86-64-v4
x86-64-v3
x86-64-v2
(none of those say supported next to them)
So it should be a fair while yet before the old x86_64 gets discontinued, although splitting it off you can guess where it's eventually headed.
The lto thing might take a bit more memory and build time for packages.