Until its weakening and ultimate repeal in 1999,是时间状语 Glass-Steagall是主语 split是谓语 commercial banking--meaning plain-vanilla lending to real people--是宾语from the riskier if mostly routine trading done by investment banks like Morgan Stanley.是状语
应该理解成 直到1999年它被废除之前 (Glass-Steagall)将商业银行业务 从 冒险业务 中 分离出来