Referential integrity checks by the DB engine (e.g. when deleting from the foreign table) require reverse look-ups of foreign keys, which would necessarily become full table scans without an index. Apart from that, applications also often do look-ups like this.

If you care about performance, they are required.

https://dev.to/jbranchaud/beware-the-missing-foreign-key-ind...

Again, another thing we learnt the hard way. All FKs now require a index for us.

I'm very curios what sort of work you do with databases to even ask this question.

Clearly you don't have the kind of "load all invoice lines belonging to this invoice" type workloads that I'm used to. Is it all OLAP?