The reason it got dropped as "unused" is that it genuinely was. In 0.0.38 and earlier, sqlmodel.main.class_registry was just this, sitting at module scope:

class_registry = weakref.WeakValueDictionary()  # type: ignore

Nothing else in SQLModel ever wrote to it or read from it (grep the old main.py, it appears exactly once), and it was never exported from sqlmodel/__init__.py. So it was always an empty dict that no SQLModel code touched. If your BigQuery compat code was reading _sm.class_registry expecting to find your mapped classes in there, it was getting an empty mapping the whole time.

The registry that actually holds your mapped classes by name (the one SQLAlchemy uses to resolve string relationship refs) is on the registry object, not the module:

from sqlmodel import SQLModel

SQLModel._sa_registry._class_registry
# same object as sqlmodel.main.default_registry._class_registry

That's the live {name: class} mapping. Verified on 0.0.39: define a table=True model and its name shows up there. _class_registry is technically private on the SA side but it's been stable across 1.4 and 2.0.

If instead you were only using class_registry as a convenient global weak dict of your own (filling and reading it yourself, not relying on SQLModel to populate it), then just declare your own weakref.WeakValueDictionary() in your module. Nothing in SQLModel was ever backing it, so you lose nothing by owning it.