After updating Ubuntu 20.04 on my Macbook Air 2011 and MacBook Pro 2014. Wifi was no longer working. Had to copy a few packages to a usb stick.
Downloaded packages from…
https://packages.ubuntu.com/search?keywords=broadcom-sta&searchon=names
https://packages.ubuntu.com/focal/module-assistant
https://packages.ubuntu.com/focal/debhelper
Mounted the usb stick and ran the following.
apt purge bcmwl-kernel-source
dpkg -i broadcom-sta-common_6.30.223.271-12_all.deb broadcom-sta-dkms_6.30.223.271-12_all.deb broadcom-sta-source_6.30.223.271-12_all.deb debhelper_12.10ubuntu1_all.deb module-assistant_0.11.10_all.deb
reboot
Reference https://bugs.launchpad.net/ubuntu/+source/bcmwl/+bug/1906745