A plan cannot be written to allow for only Roth deferrals.
1.401(k)-1:
"The term designated Roth contribution means an elective contribution under a qualified cash or deferred arrangement that, to the extent permitted under the plan, is designated irrevocably by the employee at the time of the cash or deferred election as a designated Roth contribution that is being made in lieu of all or a portion of the pre-tax elective contributions the employee is otherwise eligible to make under the plan;..."
IRS website FAQ:
"Can a plan offer only designated Roth contributions?
No, in order to provide for designated Roth contributions, a plan must also offer traditional, pre-tax elective contributions."
https://www.irs.gov/retirement-plans/retirement-plans-faqs-on-designated-roth-accounts#13
I would be surprised if any resource like EOB fails to mention this requirement.