Fish can't live without water, as/which is known to all.
這裏是(非限制性定語從句),而 as/which 替代這個句子作主語,所以 as/which 是不可省略的。假如 as/which 省略了,變成您寫的模式,句子就變成了斷句了,因為:
Fish can't live without water is known to all. 這壹句的主語是 fish,但有兩個動詞,壹個是 can't live,壹個是 is,兩個動詞之間沒有任何的連詞,就變成了斷句了,所以需要使用逗號,變成了非限制性定語從句。
As is known to all, fish can't live without water. 其實 as 本身並不是(關系代詞),因為關系代詞只有5個。假如您查考牛津詞典,as 這個單詞本身的詞性只有:介詞、連詞、副詞,但沒有(代詞)這個詞性的,也就不是(關系代詞)了。所以語法書基本上都把 as is known to all 劃入到(固定結構)中了。
Fish can't live without water, which is know to all.(這個應該是按照現代語法完整的原句)。但當非限制性定語從句提到句首時,是無法用 which 引導的,所以把 which 改成了 as,也就是把 as 的(連詞)詞性視為(關系代詞)賦予 as 有 which 的功能。這個是現代語法的解釋。as 在比如和 such,the same 連用時,也是(連詞)詞性視為(關系代詞),比如:He is such a man as I honor. 所以查考語法書時,您會發現,語法書通常使用(視為)關系代詞,而不會直接說 as 就是(關系代詞)的壹個原因了。
類似於這樣的(固定結構)還有比如:
as is said above
as is often the case
as is reported
as has been pointed out
as has been said