We are currently working on first finishing materialized view support, which will hopefully be completed in the upcoming months. Secondary indices will be implemented after that and we're hoping to reuse MV infrastructure for that. So I personally expect both features to land into a release later this year.
Please note that @glommer is talking about the classic secondary index implementation in Cassandra, which is very simple but also broken. I don't know the details but we probably did have "half-ready" code for that. We decided against going forward with it because Cassandra had already moved to SASI (which is also much more complex). As I said, we're currently focusing on materialized views, and tacking secondary indices after that.
Btw, I highly recommend subscribing to our user mailing list or engaging on Github for questions and comments about features. You'll get better and up-to-date answers there.
Update: reading @glommer's reply carefully, he explicitly says that he's unsure if we'll move forward with that specific implementation: "_if_ we do implement it, it should land in our main version in a couple of weeks" (emphasis mine).
Secondary indexes will be implemented on top of Materialized Views. Patches for Materialized Views already exist, and are soon to appear in preview releases.