I noticed hundreds of 404 error in my google webmaster portal and upon further investigation, it seems that Google is indexing based on display name rather than username.
Many are /community/subscriptions/ or /community/activity/ urls.
Is this a bug? Can I simply exclude indexing of subscription and activity URL's? I don't want those to be indexed anyway.
It seems you've disabled Guest Access to Users Activity and Subscription pages and Google see 404 error, because Google is a guest (not registered user).
That is correct. For privacy reasons, I don't allow guests to see a registered member's activity.
I recommend disable user profile indexing in Dashboard > Forums > Settings > Features admin page.
I have always disabled user profiling, so I'm not sure why it's still being indexed. I'm using The SEO Framework for my article SEO - I wonder if it's causing Google to index those pages.
I think these are old indexed data, nothing is being indexed anymore.
This is quite possible. I will wait and see if the errors go away in the future.