Two of Google's big internal databases don't use foreign keys (Spanner[0] and F1 [1]), rather they use hierarchies (parent/child tables). It definitely limits you a lot more than foreign keys, but seems to make sharding easier. It works well for some use cases, but takes some getting used to. Cockroachdb also has this functionality [2].

[0] https://cloud.google.com/spanner/docs/schema-and-data-model

[1] https://ai.google/research/pubs/pub41344

[2] https://www.cockroachlabs.com/docs/stable/interleave-in-pare...