Leading up to World War 2 (1930s etc.), what percentage of European Jews practiced "Orthodox" Judaism?
It varied tremendously across Europe. World War I actually demolished a lot of communal structure, and Orthodox observance declined from there.
Rabbi Joseph Soloveichik commented that as a young child he recalled when most Jews kept shabbos, and by the time he was a young man that had changed. As a college student passing through the great city of Vilnius, he said the synagogue was no longer that full.
(In Hassidic towns it was likely much higher.)