Ubuntu 20.04 (Focal) ships git 2.25.1 by default. git 2.25 includes `git switch` and `git restore` (introduced in 2.23) but they were still marked experimental. `init.defaultBranch` was introduced in git 2.28 — not available in 2.25. Install the git-core PPA for a newer version: `sudo add-apt-repository ppa:git-core/ppa && sudo apt install git`.