I was actually looking for a Discord extension for the Opera sidebar and was pretty surprised when I realised that Opera GX didn't have it.
But I do understand that out of all the messaging apps I've seen, Discord has to be the most resource-heavy with how many features and things it runs all at once, compared to more medium-weight messengers like Facebook Messenger and Whatsapp.
I wouldn't be surprised if users got constant crashes with Opera GX if someone decided to implement it, especially with how many times I've noticed Opera GX crashing when I have an excessive amount of tabs opened.
While I would still love for them to add or attempt it, I wouldn't be surprised if they find it to be more trouble than it's worth. ╮(￣～￣)╭