To rephrase it, I’d say, “There may be a compatibility issue after having updated Ansible but not Trellis. You probably need to bring in the Trellis compatibility fixes for Ansible 2.x”
Although you could “set up the project from scratch,” I don’t think you need to. If your project is still a git repo, you could be sure you have the official roots/trellis repo as a remote, e.g., named upstream, then…
This would merge Trellis updates into your existing project.
If you originally cloned the roots/roots-example-project.com repo instead, you could pull in the relevant updates from there.
If neither of the above (e.g., if you manually followed the steps in the example project’s README), you could git cherry-pick the Trellis updates you need from roots/trellis.
In a pinch, if you don’t have time to update, you may be able to make one edit to get past the original error you reported. Just force the string version of 'False' by adding quotes to the default here (and capitalize the letter F).
- project_subtree_path: "{{ project.subtree_path | default(false) }}"
+ project_subtree_path: "{{ project.subtree_path | default('False') }}"