I can understand that, for myself I don't miss them. An index on the column is just fine for me and personally I prefer to manage that in the application layer (I know I'm probably not in the majority). Lack of FKs is what allows for their scaling technique (using Vitess) from what I understand. I'll say for my use it's not been an issue but I do understand that migrating an app that does need/use them might be hard/impossible without other big changes in the code.