We've prepared firmware 18.104.22.16880. The AC/SHD firmware isn't present in 3.8.x, as the development for those devices has moved to the 3.9.x release branch. This is the next LTS firmware release after 3.8.12, which contains a couple important fixes. It will be pushed to the latest 5.5.x/5.6.x controllers shortly (for 1st/2nd gen APs only).
Please see below for the changelog and links to the firmware binaries.
Links are all HTTPS. This assumes your device(s) are currently running at least 3.7.51, and that the device's date is properly set (i.e. it can reach a NTP server). If you encounter issues with the upgrade, or your device isn't on a minimum supported firmware, then you can change the link to HTTP before performing the upgrade.