Huh? Logs are how you run the service. Cost is what keeps you from retaining more of them. Since you seem not to be familiar with the subject, retention policy is something of a misnomer; it would be more accurate to call it a deletion/destruction policy. The default is retain everything forever.
There are no legal considerations here. That’s what lawyers are for, and big tech has a ton of lawyers.
If you work somewhere that people are deleting things to keep them from being discoverable, run away as fast as you can.
Yes, logs are important for running the service. You were supposed to realize when you said that, that “is this useful” is a factor in how long you keep logs, alongside “how much does it cost”.
I do work at big tech and see a lot of logging policies, in fact. Having lawyers involved doesn’t make it “no legal considerations”, it makes it properly assessed legal considerations. It is default practice across big tech (and other large professional industries) to delete eg email as soon as reasonable in order to reduce discoverable records. It is also required for most services that they be able to delete logs and archived material to comply with the laws about keeping personal data around for no purpose - most places will never see this brought up but if they do, it can be a big deal.
There are no legal considerations here. That’s what lawyers are for, and big tech has a ton of lawyers.
If you work somewhere that people are deleting things to keep them from being discoverable, run away as fast as you can.