Can I connect a 5w 5w solar panel directly into a 10k mAh portable battery bank safely via USB 2.0, or does it need some sort of protection in-between like a solar charge controller or a schottky diode?
Example of battery bank (except one with usb 2.0 input): https://www.anker.com/ca/products/a1229
Example of solar panel: https://www.aliexpress.com/item/1005006012344670.html
Yes it should be fine.
You can measure and the input should become around 5v when charging