getFieldDefinitions on Drupal 8 (why all these deprecated functions on DP8?)
Drupal::service('entity_field.manager')-&gt;getFieldDefinitions(xxx, xxx);


Is the current way to get field definitions.

It's the EntityManager service that has been deprecated, not EntityFieldManager::getFieldDefinitions. A deprecation notice on a method doesn't apply to the code used within the method, just the use of the method itself.

The former currently proxies straight through to the latter, so it isn't really useful any more.

As a general rule, functions are deprecated when they're no longer useful, or when they break convention with the rest of the codebase. Don't fear it, embrace it. Progress is a good thing :)