Ok, I followed the http suggestion from@Dwaff, and Software now says it’s up to date despite these 2 pop-up alerts…
Unable to get list of updates:
failed to refresh cache: E: Type ‘"deb’ is not known on line 1 in source list /etc/apt/sources.list.d/brave-browser-release.list
Unable to download updates from"extensions.gnome.org": failed to download https://extensions.gnome.org//static/extensions.json: SSL handshake failed
When I ran cat /etc/-release* , it still lists PureOS8 green. 
Plus, as additional proof of failure, when I actually open Software, it only lists stuff from dl.flathub.org (which I erroneously added a long while ago, and haven’t figured out how to remove, sigh!).
So, I’m now trying to install Byzantium (as @Dwaff and @Quarnero advised).
Does this simply involve editing the etc/apt/sources.list from green to byzantium? Or does “from scratch” entail some other process (besides backing up my system)?
Regardless, I haven’t been able to proceed with anything due to this issue:
E: Type ‘“deb’ is not known on line 1 in source list /etc/apt/sources.list.d/brave-browser-release.list
E: The list of sources could not be read.
I don’t know how to remove these offending entries:
cat /etc/apt/sources.list /etc/apt/sources.list.d/*
“ deb [arch=amd64] https://brave-browser-apt-release.s3.brave.com/stable main”
“ deb [arch=amd64] https://brave-browser-apt-release.s3.brave.com/stable main”
@Caliga, could you please help me out? I thought I followed your Brave installation instructions correctly, but must’ve blundered.
Thanks, everyone!