AS/400 still exists actually, under the name IBM i. I know a person who swears by them. It's a fascinating system. There are no files, only persistent objects living in a 128-bit address space. Downside: still uses EBCDIC….

Actually single-level store concept was introduced in Atlas, used in MULTICS, AS/400. [1] Some modern and quite high-performance solutions are using this concept, e.g. LMDB [2]

[1] https://en.wikipedia.org/wiki/Single-level_store [2] https://symas.com/getting-down-and-dirty-with-lmdb-qa-with-s...