No, killing the caster doesn't undo the change.
An unofficial tweet from Jeremy Crawford and changes to the spell as of 2015 confirm that it "lasts until dispelled". So killing the spellcaster would have no effect.
The description of true polymorph now states:
Choose one creature or nonmagical object that you can see within range. You transform the creature into a different creature, the creature into an object, or the object into a creature (the object must be neither worn nor carried by another creature). The transformation lasts for the duration, or until the target drops to 0 hit points or dies. If you concentrate on this spell for the full duration, the spell lasts until it is dispelled.
A September 2015 tweet (albeit no longer official) from rules designer Jeremy Crawford states:
If the effect of a spell becomes permanent, it can be dispelled, unless its description says otherwise.
The way to end true polymorph is to dispel the spell somehow; killing the caster does not end the spell effect.