The instructions for adding the non-free repository are correct, except for the url, which is now/should be https://deb.puri.sm instead of https://deb.wp.puri.sm/
Although the intel-microcode package there is out of date currently It is 3.20180703.2 and still vulnerable to some of the newer spectre-like vulnerabilities.
I had to install the debian package to get the latest microcode that is supposed to fix those issues, I just downloaded it manually and ran dpkg -i to install the deb file.
(downloaded the deb from http://ftp.debian.org/debian/pool/non-free/i/intel-microcode/intel-microcode_3.20180807a.2_amd64.deb )
Manually installing that way, will definitely prevent accidently installing more “non-free” stuff.