I use the windows Slack app every day at work
and it works rather fine.
You can use f6 and shift+F6 to switch from the
messages list, to the channels list or the message edit
field. NVDA switches automatically in forms mode so that you
can type your message, and it switches back to browse mode
in other situations.
With the automatic forms mode switching you can
navigate through the messages with up and down arrow keys.
Sometimes NVDA gets stuck because it cannot
decide if enabling forms or browse mode, but in most
situations, it works rather good.
There is a useful help for you to learn how to
use Slack with a screen reader :
Use a screen reader on Slack on
desktop, that provides helpful information.
I have to agree with the other person who
replied. It’s absolutely doable but it’s a bit cumbersome at
times. Once you get familiar with its quirks you do get used
to it, and ultimately every area of the app is accessible.
[Edited Message Follows]
The Slack Windows client is fairly usable with lots of hot
keys for faster navigation. Those that I use the most are
ctrl+k and ctrl+j. It is annoying at moments and never
pleasant, but covers my needs and most likely yours.