These are the seven days of mourning for the righteous man Methuselah for whose honour the Holy One, blessed be He. had regard, and therefore postponed punishment. Go and calculate the years of Methuselah and you will find that they came to an end (i. e. he died) in the six hundredth year of Noah’s life (which coincided with the date of the Flood) (Sanhedrin 108b).
Rashi calls him a Tzaddik. This answer cites (without real source) the statement that there were
seven long-lived saints whose successive lives extend over the whole history of mankind; each having transmitted the sacred lore from his predecessor to the one succeeding him, while shielding the generations of his time by means of his piety. These saints are: (1) Adam; (2) Methuselah;
This answer discusses sainted individuals in the context of belief in God and lists him.
Whereas this Q/A lists him (according to some opinions) as a "shepherd".
If he was such a tzaddik/saint/shepherd/believer, so much so that Hashem delayed His actions to respect his mourning period, why was he not saved with Noach (or why was he not the one who was spoken to 120 years earlier and warned about the flood)?