Existing Primer ViewComponents behaviours don't use Catalyst or any other dependency, while GitHub developers often start writing a component using Catalyst.
Removing the Catalyst dependency from the Web Component is an extra burden on Web Developers with few benefits. The library is ~1kb of JavaScript, and github.com already includes the library as part of its client-side bundle. Adding the library to Primer ViewComponents does not affect github.com and has a minimal effect on other applications using Primer ViewComponents.
Decision
Any new Primer ViewComponent behaviours should be authored using Catalyst.
Considered Options
Don't use Catalyst in PVC.
Pros:
Developers who don't have Catalyst experience don't need to learn how the library works.
The bundle size doesn't need to increase by 1kb.
Cons:
Migrating components from the github.com codebase would need to be converted to Web Components without Catalyst.
Use another JavaScript behaviour library or strategy than Catalyst.
Pros:
The bundle size doesn't need to increase by 1kb.
Cons:
Migrating components from the github.com codebase would need to be converted to Web Components without Catalyst.
PVC JavaScript behaviours would differ from how GitHub engineers write JavaScript behaviours today.
Consequences
The Primer ViewComponents JS bundle gets larger.
Developers unfamiliar with Catalyst need to get familiar with the library.