See below, the requirement for a safe harbor notice, and I assume the removal of a safe harbor notice, is a facts and circumstances test, it must be a reasonable time period before the plan year. Whether 11 days was a reasonable time period, is a facts and circumstances determination. Basically, I would ask whether it gave the individuals a long enough time to change their deferrals, or alter there behavior based on the change in the plan.
https://www.irs.gov/retirement-plans/notice-requirement-for-a-safe-harbor-401k-or-401m-plan#:~:text=General rule%3A Generally%2C the safe,beginning of each plan year.
"General rule: Generally, the safe harbor notice must be provided within a reasonable period before the beginning of the plan year. The timing requirement is deemed to be satisfied if the notice is provided at least 30 days (and not more than 90 days) before the beginning of each plan year. If the notice is not provided within this time frame, whether the notice is timely depends upon all of the relevant facts and circumstances."