分析从句Then very rapidly a situation was reached where COVID-related hashtags occupied 30–70% of the HSL, however, with changing content.

这个句子包含了一个主句和一个含有with短语的从句。主句是“Then very rapidly a situation was reached”，表示一种情况迅速到来。从句是“COVID-related hashtags occupied 30–70% of the HSL, however, with changing content”，其中with短语“with changing content”作为从句的补充说明。这个从句描述的是这种情况是指与COVID相关的hashtag在HSL中占据了30-70%的比例，但是它们的内容不断变化。