water under the bridge vs bygone

water under the bridge and bygone both are nouns.

Word	Noun	Adjective	Verb	Adverb
water under the bridge	Yes	No	No	No
bygone	Yes	Yes	No	No

As nouns, water under the bridge and bygone are synonyms defined as:

water under the bridge (noun)	bygone (noun)
past events to be put aside	past events to be put aside

water under the bridge (adjective)	bygone (adjective)
	well in the past; former