I found this article for you that organizes the IRS Q&A nicely:

https://rsmus.com/what-we-do/services/tax/compensation-and-benefits/employee-benefits-programs/irs-u...

The only requirement I can think of that meets what you seem to be asking is the one for how large a staff the employer had in 2019 vs the impacted year which is the latter 3/4 of 2020. 2019 is not the impacted year.