Ferroelectric nanocomposites have great potential applications and are in the focus of modern studies. Their polar properties are due to spontaneous polarization in nanoparticles confined to insulator matrices. The crucial problem is the persistence of ferroelectricity under nanoconfinement as the ferroelectric phase transition can shift because of size effects and other grounds. We report the 31P nuclear magnetic resonance studies of nanoparticles of well known ferroelectric potassium dihydrogen phosphate (KDP) embedded into silica opal matrices. Two NMR techniques, static and magic angle spinning (MAS), were applied to confined KDP particles with different levels of deuteration, 80% and > 95%. Measurements were carried out using a Bruker Avance400 pulse spectrometer. Our findings proved unambiguously the pronounced reduction of the ferroelectric transition temperatures in the deuterated KDP/opal nanocoposites compared to the bulk counterparts in striking contrast to previous results.