dbt 1.5+ supports primary_key and foreign_key constraints. However, as of dbt-core v1.5.2, constraints require contracts to be enabled. The catch-22 is that contracts then require ALL model columns to define data_type, but data_type is database-specific, so you lose the incredible database agnostic features of dbt. For this reason, we use the dbt_constraints package to enable cross-database support for primary and foreign keys.

This issue has been raised in the following dbt-core discussion: dbt-labs/dbt-core#6726