Until three months ago, distributions were *forbidden* by Intel from redistributing Intel's microcode. That's one reason why almost none of the distros -- including Slackware -- update Intel microcode.
https://www.theregister.co.uk/2018/0...ocode_license/
https://perens.com/2018/08/22/new-in...ot-acceptable/
https://twitter.com/imadsousou/statu...-License-Fixed
Another reason is that, in almost all cases except Skylake, there are better fixes in the Linux kernel that do not rely on Intel's microcode.
More reasons include that Intel publishes no information about what's in the microcode updates, and Intel sent out a lot of broken updates at the start of this year.
You can build and install Intel microcode for yourself from
SBo, and there is support for that in Slackware's mkinitrd package.
You didn't ask, but fyi, AMD's microcode is redistributable, and comes in Slackware's kernel-firmware package.