Limitations of Predictive Indexing

Here's a reply from a product manager working on large list updates:

Predictive indexing applies to lists between 2500 and 20,000. End-users may also manage indexes in lists as large as 20,000.

If a large number of items is created in a short time, whether by third-party tool or some other means, it's possible that the predictive indexing may miss the chance to manage indexes for the list. After the list has grown larger than 20,000 items, predictive indexing will no longer make changes to it.

Predictive auto-indexing is enabled by default. It may be disabled for a specific list in Advanced settings.