Hey Benny!

 BP> only 6.4.0 here on gentoo is stable

I haven't used that version in awhile.  My best guess is that it is perfectly
fine.

 BP> i dont know why older kernels stopped building firmware

There is little need to rebuild firmware unless of course you are adding new
hardware that requires newer firmware.  If that is the case then the older
kernel's in-tree firmware is unlikely to resolve that issue.  All of the
firmware I currently require is supplied by linux-firmware.git instead so it is
 of no consequence that it is no longer supplied by the newer kernels
especially considering none of it was ever used in my particular situation. 
Exactly what firmware did you need to build with every kernel upgrade?  I bet
very little, if any, has changed as far as firmware is concerned especially
regarding an i686 based system.

 BP> have it build in kernel makes sure its tested and works with the
 BP> kernel version

For example?  The newest update here was on Nov. 26, 2017 (htc_9271-1.4.0.fw)
and was never supplied by the in-tree kernel's firmware.  It was always
maintained by linux-firmware.git.  So far I've never needed to obtain firmware
from any other source ... thank goodness.

Life is good,
Maurice

... Don't cry for me I have vi.
--- GNU bash, version 4.4.19(1)-release (x86_64-silvermont-linux-gnu)
 * Origin: Little Mikey's Brain - Ladysmith BC, Canada (1:153/7001)