Based on what you stated, it seems like you want to know if offloaded apps will still use battery. Offloaded apps are no longer on the iPhone to save space. Your documents and data are kept though. Since they are not on the iPhone they should not be using the battery.

Does closing apps save battery?

Does Closing Background Apps Save Battery? No, closing background apps does not save your battery. The main reason behind this myth with closing background apps is that people confuse 'open in background' with 'running.

Is it good to enable offload unused apps?

When you use the Offload Unused Apps feature, your iPhone will automatically delete apps that you don't interact with much when you're running low on storage. Normally, it's a great feature that helps you manage your storage, but if you have a COVID-19 contact tracing app, you might want to turn it off.