The threading documentation for 3.9 still claims "While they are not listed below, the camelCase names used for some methods and functions in this module in the Python 2.x series are still supported by this module." It would be better to mention when isAlive was removed.

The method is still used in some major libraries, like NLTK (https://github.com/nltk/nltk/blob/637af5380d6071517a5f0d224649e5c3560b5f91/nltk/inference/api.py#L536). Documenting the removal clearly in the threading docs would make it easier for developers to upgrade.