[KeyBoardG]

Unless your project is small and for learning, or won't be around for very long, Yes. You absolutely do or will end up in a technical debt world of hurt. I have worked on systems without them that are 15+ years old and the vast, vast majority of fixes and refactoring were self inflicted wounds like this. We added FK's and indexes where necessary and wound up faster and safer.

[x86_64Ubuntu]

That sounds awful. My first instinct would be to see where an FK constraint would fail, and then ask the business what it should be. There is no guarantee that every business case from way back needed an FK, or that anyone in the business knows which FK value should be there. It sounds absolutely miserable.

[eddd-ddde]

In my experince, adding proper constraints will not only ensure code can never write invalid data to disk, it will also help you realize when business rules are lacking or failed to consider edge cases.