> FK constraints obviously dont work across system boundaries.
Not to their full extent, but they can still be used. At the simplest level, it is of course entirely possible to give different services their own schema in a given database, and FK constraints are supported across schemata in both MySQL and Postgres. Vertical scaling can take you enormously far with properly architected schemata and queries.
A more flexible, but still easy to reason about way to accomplish this is to have local versions of certain tables in each DB. This can be manually implemented (though this is not an easy problem to solve, for a variety of reasons), or by using something like Citus [0], which accomplishes this using 2PC. This is of course slower, but if your data model is carefully designed, it can be managed.
> soft deletes.
Sure, but now you have a new problem - needing to add a `is_deleted` or `deleted_at` column to a bunch of tables, and indexing that column on every table. In Postgres you might get away with this by using `DEFAULT [FALSE, NULL]` (respectively), and then creating a partial index with `...WHERE <column> IS NOT [FALSE, NULL]`; that way the index size stays reasonable, and the cardinality isn't as horrible (well, it is for bools, but since you're only using it as a filter, it can work OK). Also, of course, you have to include this predicate in most queries.
> Its theoretical protection, not practically needed IMHO.
Different subjective experiences, of course, but IME it very much saves you time and headaches.
[0]: https://www.citusdata.com