6379.txt (and branch) replaces index_interval with min_index_interval and max_index_interval. When migrating, the existing index_interval value is used for min_index_interval. I chose 2048 for the default max_index_interval; in practice, this limit should only be hit for infrequently-read SSTables when the index summary memory pool is very full.
If you want to test it out, I suggest setting logging to TRACE for o.a.c.io.sstable.IndexSummaryManager, setting index_summary_capacity_in_mb to 1, and index_summary_resize_interval_in_minutes to 1. After inserting a few million rows with stress, the index summary pool will hit capacity and some resizing will happen. The JMX functions on IndexSummaryManager are also handy.