We’re running a forum at https://forum.fp2.dev/ which currently requires signing up/logging in to view or post any content.
We want to make the content indexable to search engines and viewable to users, only requiring an account if they want to post.
This form here itself seems to have that functionality already enabled but I can’t figure out where the setting/config is. How is this enabled?
The setting is called login required:
Require authentication to read content on this site, disallow anonymous access.
Assuming you aren’t a CDCK customer on the Basic plan you will find it in /admin under Settings → Login.
By enabling enable sitemap your site will offer a sitemap at https://forum.fp2.dev/sitemap.xml which can be submitted to the Google Seach Console to encourage indexing.
I don’t think this will work. You will see a login page before accessing the content. The sitemap won’t be accessible too. 
That’s why they’ll be changing it from on to off:
What OP described means that setting is currently enabled. Looking at the link they provided that site is currently login only.
Turning it off will both allow content to be indexed and viewed by users. They will only need to register and login if they need to post.
Right, I’ve completely understood the wrong way. Maybe it’s time for me to go to bed, sorry! 
Ah, this worked perfectly. Thank you!