For instance, in the prehistory of Modern Persian, apocope introduced word-final clusters of obstruent or nasal + r.    From Wordnik.com.  [Archive 2008-07-01] Reference

This led to terminal obstruent devoicing spreading to the rest of the continuum, and I suspect that þd may also have originated with the Franconian node though this is little more than speculation.    From Wordnik.com.  [The PIE and Pre-PIE pronominal system from the perspective of a wave model] Reference

If I'm correct about terminal obstruent devoicing continuing in the region, then they never did speak Old High German as we usually think of it note that terminal obstruent devoicing is one of the criteria for distinguishing MHG from OHG.    From Wordnik.com.  [The PIE and Pre-PIE pronominal system from the perspective of a wave model] Reference