I think they used to index everything they found. But WWW has grown incredibly during the past 10-15 years.

They will crawl almost everything (where not explicitly prohibited), however, indexing is dependant on several factors and not guaranteed.

I would personally expect them to index all public GH issues, though.

Google doesn't index everything and they don't even return results from their index with perfect precision. You have to make some trade-offs at that scale.