As much as I love it not every app needs it :)

Depends on the product. If you’re trying to get an MVP out the door, don’t do it.

Yes. The latest operating systems include dark mode as a system wide feature and should be supported by all new projects.

We're building a web chat app and seems we have to have dark mode; lest evolved users think we don't care enough for good tech (low battery impacts etc.) Poll option of budget is not valid I feel because if you use the right framework it can be quite easy to implement. And of course not every app needs it, but if you are a web chat, analytics or anything that needs to be on for long, consider dark mode.