Secure 2- Retroactive adoption of deferral contributions

Secure Act 2 permits sole proprietors (and LLC taxed as a sole proprietor) to adopt a 401k plan including salary deferral contributions effective retroactive to the first day of the prior tax year up to the due date for filing the sole proprietor's individual tax return for the prior year. Why didn't this section of the act include partnerships? Can a partnership adopt a 401k plan retroactive to the prior year and allow partners to make retroactive salary deferral contributions?

A partnership cannot retroactively adopt a 401(k) Plan.  I've heard a few speakers bring this up, and it appears to have been an oversight.  Nevertheless, there you are.