In version 12.6.0 of bloomreach, the cms version is 5.6.0. In a managed dependency, '‘hippo-cms-richtext-frontend,’ I no longer see HtmlCleanerPlugin, which I see in version 3.2.1. How can I find this class?

In version 12, the HTML cleaner plugin has been replaced by a new HTML processor service. The configuration also moved to a new location in the repository.
See for details.