I can confirm the package from Slackware current, as of today, seems to work fine on Unraid 7.0.0
https://mirrors.slackware.com/slackware/slackware64-current/slackware64/ap/powertop-2.15-x86_64-2.txz
I can get down to C8 like before.
As a side note, I also tested running powertop on a privileged Ubuntu container and it seems to work fine as well, adjusting everything on the host system