How does this work in terms of bots crawling for indexing?
Am I right in saying as its just CSS the bot will still be able to index the information behind?
Those bots should not index at all. If bots can do it is very bad design flaw that can backfire very easily.
Search results aren’t made for free advertizing. Search results are made for finding things from web content. And Google will punish fast and hard if they find content that is free bot only and targeted to bypass free content.
Good news is that Discourse doens’t do things like that. If visibility is another than everybody that content is forbidden for bots too.