Can a plan sponsor terminate a source of the plan?

Example-  I have an ESOP plan that includes a 401k deferral feature.  The employer would like to remove the deferral feature, including all the deferral assets and close the accounts.  Is it possible to write the partial plan termination amendment to terminate only the deferral portion of the plan that would create a distributable event for the participants?

No.

You don't identify your precise role.  A consultant (i.e., someone more than a TPA) would ask questions to help the sponsor determine what is the underlying concern (which may or not be a problem), and then help the sponsor look for a solution, perhaps alternatives to consider.

There may be other complications I'm not thinking of off-hand, but could you spin-off the 401(k) portion into a separate plan, then terminate the 401(k) plan?

The ESOP would not be a successor plan.

I've done the opposite - added a new 401(k) to an existing ESOP by creating a new KSOP then merging the ESOP into it - and received a DL with the IRS treating the KSOP as a new plan. (This was after the elimination of the DL cycle, so the plan had to be new to get an initial DL.) Seems like the OP would be the same transaction in reverse, i.e., "un-merging" the two components into separate plans.