As identified in SERVER-67924, applications that use the 4.0 accepted command

db.getSiblingDB("test").runCommand({ collMod: "test_col", usePowerOf2Sizes: <boolean> });
or
db.getSiblingDB("test").runCommand({ collMod: "test_col", noPadding: <boolean> });

will crash any 4.2 secondaries that attempt to replicate this command.

This can lead to an undesirable upgrade experience that can cause 4.2 secondary nodes to suddenly crash while following the upgrade procedure.

We should recommend that users remove all instances of collMod that uses legacy options in their application.

1. https://www.mongodb.com/docs/manual/release-notes/4.2-upgrade-replica-set/
2. https://www.mongodb.com/docs/manual/tutorial/change-replica-set-wiredtiger/

Note that we already document these options as deprecated in our existing collMod documentation:

MongoDB removes the MMAPv1 storage engine and the MMAPv1 specific options noPadding and usePowerOf2Sizes for collMod.