Amend SH Plan now to exclude class?

Can I amend a 2019 3% SH plan now to exclude a certain class of Employee.  Coverage not a problem.  None of the current Employees thusly situated are eligible for the plan now. 

If they are not currently eligible, you can.  See Notice 2016-16 Section III D 2, last sentence.  If they are eligible, you can't.

Recall that the 1.401(k)-3 provides requires that all "eligible employees" who are not HCEs must be in the ADP SH arrangement, and that 1.401(k)-6 defines an eligible employee (for that purpose) as anyone eligible to defer.   (Whoever would be in your ADP test needs to be in the ADP SH, a big deal for a nonelective ADP SH contribution.) 

Your only escape, if the document permits it, is the disaggregation of a group that can be permissively disaggregated under 410(b) (e.g., otherwise excludable employees, collective bargained employees, QSLOB, etc.).   So it doesn't matter that you can satisfy 410(b) coverage if you can't satisfy 401(k) because you are setting up a class exclusion like "all janitors" or all "legal associates."   You could, however, for example, exclude legal associates who are HCEs (subject to any limitations imposed by a particular preapproved plan, such as (perhaps) including or excluding all HCEs).

Notice 2016-16 rests on the foundation of the 401(k) regulations (and so states in a couple places).  Nothing in that Notice supersedes any regulatory provision.  That Notice merely clarifies what may and may not be amended midyear.  The Notice might permit a midyear amendment at this time, but the Notice doesn't confer the ability to amend in any way that would violate the 401(k) regulations, i.e., you might be unable to retain such a document design beyond the point that any of those employees must be treated as "eligible employees" for purposes of the 401(k) regulations.

It may be that the only way to permanently exclude a particular group from the ADP SH might be to exclude that group from eligibility to defer, and <that> would be subject to 410(b) coverage testing for the K component of the plan.

^ Thanks, Doc.  I wasn't planning on excluding them from just the SH.  I'm going to exclude them from the plan altogether.  deferrals, SH and PS.  It should pass coverage.