Decades of merging flats and multitenant houses into larger residences have silently depleted New York City’s housing supply, particularly in Manhattan, according to new research. Such consolidations have eliminated 104,000 housing units since 1950, offsetting gains from new construction. “What this illustrates is that you just have to build a lot more housing. You have to make up for the fact that people are doing this,” said Adam Brodheim, historic preservationist and author of the study.