It is just a warning, you can ignore it. Write API should be compatible, I will update the compatibility string 🙂
Is there an option to create charts based on specific data? I'd like to try create kind of training diary, where I could see progress of specific day/weeks/months of training with some interactive charts, is it doable in NodeBB?
You can do pretty much anything you want if you're willing to put in the work to make a plugin. Without that, though, no there's no way to configure NodeBB core to do that.