Anything you change here will most likely be over written by the Slackware kernel-firmware package, because the SARPi package conflicts with it.
WiFi worked fine on my RPi4 when I tested the latest official Slackware install image about a month ago but I didn't test it extensively for long periods, nor saturate the interface.
If there's an issue with WiFi on the RPi4 or 5 let me know so I can update it.
The script we use to pull the firmware is
here, so you can see which version we're using.
I don't know about any firmware for the RPi5 yet, but if there's some additional firmware I can bundle it in there.
Cheers