Package: linux-image-4.19.0-6-rpi
Version: 4.19.67-2
The linux-image-rpi kernel for armel does not include CONFIG_BRCMFMAC_SDIO=y. This config option is necessary to support the WiFi on the Raspberry Pi Zero W. I have verified that enabling this option fixes the problem on my Pi Zero W by building a version of the kernel package with the option enabled following
https://wiki.debian.org/HowToCrossBuildAnOfficialDebianKernelPackage.