Basically, if the addon wants to interact with any kind of urls, this message is unavoidable. Which means that even if the addon doesn't require to access any data of the websites, as long as it wants to be triggered for any websites, this message is not going to be avoidable.
But GeckoView still uses Gecko, it's not like a new engine which built from scratch. It's like a wrapper of Gecko, which removes a bunch of things from Gecko.
Basically, if the addon wants to interact with any kind of urls, this message is unavoidable. Which means that even if the addon doesn't require to access any data of the websites, as long as it wants to be triggered for any websites, this message is not going to be avoidable.
https://extensionworkshop.com/documentation/develop/request-... has more information.