as引导的时间状语从句中的be动词

It began, and for many years continued toexist, as a supplement to the apprenticeship system still in vogue during the seventeenth and eighteenth centuries. 请问老师为何这个句子中still前没be动词，as引导的时间状语从句难道可以省略be动词吗？谢谢老师oo

as a supplement to the apprenticeship system still in vogue during the seventeenth and eighteenth centuries

—— 总体是个介词短语。as 是介词，as a supplement 表示句子主语 It 的身份，或者作修饰谓语动词的“方法”，不是“时间状语”。

1. to the apprenticeship system：介词短语作supplement 的定语。

2. still in vogue during the seventeenth and eighteenth centuries：介词短语作system 的定语。

3.during the seventeenth and eighteenth centuries：作in vogue（时尚）的时间状语。