Even discord and slack I don't want browser notifications. Inside their own tab, I can deal with ui elements. There's no need nor no want to have a page alert outside of the web page itself.
It can be useful when you get mentioned, like for example, a user has pinged the moderator role where a quick reaction will be necessary. Desktop notifications can significantly reduce reaction time to important events (though, assuming you do the sane and set Discord to only notify you on mentions that aren't @here or @everyone)
Well, if you don't know me well enough to have my phone number, you certainly shouldn't have the privilege of making my computer beep at me, whenever the hell you feel like it. But, that's me. Maybe you like people interrupting you. (Or maybe you like interrupting people because you think you are too important to wait en queue.)
The discord server I moderate has a strict "don't ping moderators unless it's important" rule and in general anyone who just pings for shits and giggles gets quickly muted.
I like it when people interrupt what I do when the server I moderate and therefore are responsible for is atm being swarmed by people spamming senseless shit into the chat or someone going of the deep end and yelling racist slurs, thank you very much.
I'm not suggesting your position is wrong, just that like IRC there's going to be lots of discord users who have a use for some kind of notification but would never want to give their number out.