Broadcom BCM94331 (14e4:4331) uses HT PHY but let us stick with the logic (solution) or philosophical approach (idea), my favorite source here, coming from from @purple (post #26), still not promising it will work while not using PureOS (yet) on MBP. As well, this approach will modify PureOS kernel (stay aware if future kernel updates recognize upcoming change) and therefore this post should be flagged (if so I’ll sing: “Let it Be” from The Beatles).
By rewriting few things, already mentioned ones, please follow this (draft) within Terminal (using Ethernet connection):
~$ sudo apt purge broadcom-sta-dkms
~$ cd /etc/apt
~$ gksudo gedit sources.list or sudo nano sources.list
Add following line: deb http://deb.debian.org/debian buster non-free
~$ sudo apt update (NO_PUBKEY error might appear), if so:
~$ sudo apt install debian-ports-archive-keyring
~$ gpg --keyserver subkeys.pgp.net --recv-keys B4C86482705A2CE1 (if correct, by comparing it to previous: apt update output)
~$ sudo apt-key update
~$ sudo apt update
~$ sudo apt dist-upgrade (hopefully without significant changes, if any)
~$ sudo reboot
~$ sudo apt install wireless-tools network-manager-gnome wpasupplicant
~$ sudo apt install dkms linux-image-$(uname -r|sed ‘s,[^-]-[^-]-,’) linux-headers-$(uname -r|sed ‘s,[^-]-[^-]-,’) broadcom-sta-dkms
~$ sudo modprobe -r b44 b43 b43legacy ssb brcmsmac bcma
~$ sudo modprobe wl
~$ sudo reboot
If BCM4331 chip works and desired: remove or block (with ##) from sources list DebianBuster non-free repository.
This above is just simple draft, not PureOS related, another choice (needs to be proofed and hopefully needed modifications posted here), that might turn on Wi-Fi, yet not compatible (in broader terms) with PureOS. If tested, someone may recap what works/needed and what isn’t working. In addition, post #74 (open source b43-fwcutter driver) might be obsolete, but not sure.
I like aptitude very much:
~$ sudo apt install aptitude
EDIT: sudo apt purge <package_name> Thanks, @ruff!