Why “God remembered Noah” (Gen 8:1)? Had God forgotten him?

Of course not; that God had forgotten Noah is not at all the force of the statement. Rather, the force of the statement is to assert for the benefit of the faithful reader that God could, in this case too, be counted upon to do what he promised. Throughout the Bible, in the same way, God “remembers” (זָכַר, zakar) the promised blessing of many. For example, in Gen 19:29, God “remembers” Abraham before destroying Sodom, and so brings Lot out of Sodom; Abraham had extracted a promise from God to save Sodom even for 10 souls (Gen 18:32). So he was willing to stay his hand as Lot’s family had the opportunity to leave. This is one of many examples.