If it's dangerous, note it in the class/method Javadocs, don't just blindly slam the door shut.

— extensibility by adding new features and services through management of groups of DOI names,

Yes, using custom tags. Example in Python, making the !join tag join strings in an array:

For pluggability, an endpoint is split into a Protocol and an Adapter part. This separates generic logic from environment-dependent code.

Note how a handful of default steps lead into six standardized termini, allowing to plug protocols into different adapters. Imagine replacing your self-written API adapter with a canonical JSON-API adapter, for example.

Configurable syntax! You can add new rules and even replace existing ones.

Community-written plugins and other packages on npm.

With the new plugin based architectue, the library has really small core. Everything else is built around as a plugin.