The right to self-determination seems to me to have been "recognized" as propaganda, but practically never practiced.

It was used post WW1, but only because there were two big multi-ethnic empires to be broken up. No-one proposed treating the victors similarly; their constituent nations which wanted independence had to fight for it, like Ireland did in 1920-1922.

Very few significant new nations have claimed statehood in the century since then on the basis of this principle without armed struggle. And when there's a civil war or rebellion and one side wins independence by military and political means, I don't give much credence to abstract principles.

Using Wikipedia's list of sovereign states by date of independence for the last century, the only states in the first half of the list (from 1973 to the present) that were established peacefully along ethnic lines are Czech and Slovak republics in the post-USSR breakup of Czechoslovakia. Most other Soviet states became nations despite being multi-ethnic, or fought bloody civil wars as in Yugoslavia. So did almost all African and Asian colonies post decolonization.

I admit I didn't have the patience to read all the linked articles on that list, and its older half (1914-1972), but at least its first half doesn't contain a single example of a part of sovereign nation breaking away on the basis of self determination without a major war. The older half probably might have some examples, but I expect them to be very few.