Given that threads are identified by slugs in the URL, I wonder how collisions will be handled
They won't, GOG will end up trashing them (probably overwrite, maybe merge) because they have a long history of not thinking things through before they do them and inevitably breaking everything.
Well done GOG, you've broken every link to a GD topic that was created before 2015 (both on and off site).
I wish you'd think before you did things like this.
Not only that, this is a potential security risk. Someone could create an account that looked similar to an old account, create a topic with the same name as an archived one and now all the pre-2015 links will point to their MitM topic instead of the original.
For example, any links to that originally went to here
now go here instead
EDIT 2018-03-29: Fixed post because GOG broke the links somehow.. seems to not like multiple urls in a single line.