It still isn't possible to package Tailscale or anything that creates a virtual interface as a Flatpak because there is no permission for that. macOS has an API to ask for permissions to add an interface/change routes.
Tailscale is more like a system service that I'd prefer a distro package for (Arch Linux repos contain Tailscale, btw).
It's possible but not ideal. The application could use flatpak-spawn (to get out the sandbox) and then polkit-exec (to ask user for root perms for arbitrary use) to get root privileges on the host, but you're removing nearly all sandboxing.
To me, Linux doesn’t have a granular modern permission system, and I don’t expect my package manager to solve it for me. I still run proprietary software on it, because I kind of have to. Is that an ideal situation? No. But I rather have a distribution system and vet vendors (which I’m doing anyway) than wait another decade for permissions to be perfect. Distribution, packaging and updates is the pressing need imo.