According to NameVoyager, which is a visualizer based on US Census data, the name “Adolph” was in decline from as early as 1880, which is the earliest for which data is shown. By the 1970s, it was no longer in the top 1000. But, and here’s the odd thing, in the 1940s there was a noticeable decrease in its rate of decline.

See it in more useful detail here.

Popularity of names starting with ADOLPH