This package includes outdated registered functions for spaCy v3.x, for example model architectures, pipeline components and utilities. It's installed automatically as a dependency of spaCy, and allows us to provide backwards compatibility, while keeping the core library tidy and up to date. All of this happens under the hood, so you typically shouldn't have to care about this package.
Whenever a new backwards-incompatible version of a registered function is available, e.g. spacy.Tok2Vec.v1 → spacy.Tok2Vec.v2, the legacy version is moved to spacy-legacy, and exposed via entry points. This means that it will still be available if your config files use it, even though the core library only includes the latest version.
Legacy functions are exposed with the prefix spacy-legacy, e.g. spacy-legacy.Tok2Vec.v1. When spaCy resolves your config and a function is not available in the core library, e.g. spacy.Tok2Vec.v1, it will check if there's a legacy function available and fall back to that. You can also explicitly refer to legacy functions in your config, to indicate that a newer version is available.