In Azure Devops browser(pipeline yaml editor),
Ctrl + K + C Comment Block
Ctrl + K + U Uncomment Block
There also a 'Toggle Block Comment' option but this did not work for me.
There are other 'wierd' ways too: right click to see 'Command Palette' or F1
Now it is just a matter of #
or even smarter [Ctrl + k] + [Ctrl + c]