> That's what I did: I found a 2017 stage3 with a still older glibc and
> managed to upgrade to a 2020 gentoo while masking the last glibc
> versions. That was tricky because I had to git-checkout intermediate
> versions of the portage tree in order to deal with the EAPI changes but
> I have a working chroot now. Thanks.

That's the easy way to do it, yes.

The hard way is to treat this as a cross-compilation problem and bootstrap 
your own stages from scratch. Instructions would be a bit longer...

-- 
Andreas K. Hüttel
dilfridge@gentoo.org
Gentoo Linux developer
(council, toolchain, base-system, perl, libreoffice)