For an experiment, I loaded Meta using the crawler view using the GoogleBot user-agent string. Then went all the way to the page 666 of our latest list, which are topics last bumped in mid-2017, almost 3 years ago.
One of the topics in the list is Deep-integration of Discourse within an Ionic App. I went to a not logged in Google search and the search for “integration discourse ionic” puts it on the very first result!!
Meta is a “small” instance with under 30k topics, but all appear to be indexed properly. Since it is an old domain and we are the #1 result for all things Discourse, we get enough “karma” with GoogleBot, so it runs on our domain for a sufficient time to crawl everything that is needed.
Did your forum went a migration from old software to Discourse?