It is more than just kernel. All core packages are built to use certain CPU instructions that older CPUs do not have.
With RHEL this did start at 9: El9 will require x86-64-v2 support and Building Red Hat Enterprise Linux 9 for the x86-64-v2 microarchitecture level | Red Hat Developer
As stated there, you can run (on systems that have recent glibc):
/lib64/ld-linux-x86-64.so.2 --help
and see which microarchitectures your hardware supports.
el9 requires x86-64-v2 (SSE4.2, etc)
el10 will require x86-64-v3 (AVX2, etc)
AlmaLinux seems to build additional el10 with x86-64-v2 requirement (but without third-party packages that aint much). That is, different distros can build their packages from same sources, but with different compiler options. More-over, distros like RHEL do tweak their sources. I don’t know whether the warnings about “deprecated, unmaintained, etc” are from Red Hat or from upstream.