This is more a note for my benefit — for search index update strategies, the onPublishEndAsyncSingleInstance’ makes the ‘onPublishEndAsync’ a deprecated option.
The legacy onPublishEndAsync remains to ensure backwards compatibility, but from Sitecore 9.0 onward it’s the default index update strategy used by Sitecore.
With that said, it appears in Sitecore 9.0 update-2 there’s a major defect with OnPublishEndAsynchronousSingleInstanceStrategy. The ContentSearch.ParallelIndexing.MaxThreadLimit setting is ignored by the onPublishEndAsyncSingleInstance strategy — so incorrect thread limits can be used (slow perf!). Sitecore’s patch reference # 285903 can be requested through Sitecore Support to address this.
I suppose it’s a consequence of the new onPublishEndAsyncSingleInstance not having a mature and well-tested codebase surrounding it (onPublishEndAsync has been around for ages!).