Yes, there is probably many extensions that make outside connections that don't add benefit to the user by doing so.

I wish I could block per-app connections on Linux like Little Snitch appear to allow on Mac.

Isn't Little Snitch essentially an interactive firewall? Rather than silently denying/allowing traffic, it needs the user's decision until a connection is white/black listed? Why would this not be allowed on Linux? (other than the app doesn't exist, yet)

Every once in a while something comes up trying to be 'Little Snitch for Linux' but none has survived AFAIK, To be honest one of the reasons I use macOS is for LS and I've heard few others say that too. But now since macOS is bypassing LS or limiting its function or to put it simply doing weird network stuff I'm planning to get back to Linux.

This is entirely possible. Either by isolating the application into a network namespace (e.g. via firejail or systemd units), with selinux labels, running the process under a custom gid and various other mechanisms.

Anything is possible, but is it relatively easy to block all apps and keep a whitelist of allowed apps?

I have not done that on a desktop but seen it on servers with selinux, each service we added had to be labeled properly to get network access, one extra line in the deployment script. I'm not aware of an GUI tools though, if that's what you're asking. I think that's also the approach android uses to enforce app permissions, they obviously have a gui but that doesn't integrate with normal desktop environments.

a single line in a deployment script is as simple as it can get, thanks, I will need to give this a try

Extensions used to be able to even network requests triggered by browser internals and by other extensions. I think with webextensions this is no longer possible.

Correction: I meant 80/443 usually gets allowed for entire browser on Little Snitch, so browser extensions dialing something gets through that.