Short answer: Yes, search engines can index the content after enabling the Sign-up wall.
Sign-Up Wall2 followers4 questions1 post
Do you have questions about Sign-Up Wall?
Log in to ask questions about Sign-Up Wall publicly or anonymously.
Dear @Jonathan57 , after reading your question I decided to look further and here's what I've found.
In short, it seem to be design to encourage visitors to sign up by limiting the amount of content they can see as anonymous visitors of your page.
The sign-up wall is a feature released on our App Store. By installing this app, visitors would need to sign-up to be able to see the content fully while the content is being indexed by Google.
Therefore, when you publish interesting content on your community, people would not just read it and leave. They will need to sign-up into your community to read the content.
Please click here to learn more about this feature.