Tiering didn't go away with the migration to all-SSD storage. It just got somewhat hidden. All consumer SSDs are doing tiered storage within the drive, using drive-specific heuristics that are completely undocumented, and host software rarely if ever makes use of features that exist to provide hints to the SSD to allow its tiering/caching to be more intelligent. In the server space, most SSDs aren't doing this kind of caching, but it's definitely not unheard-of.