lang syne vs long since

lang syne is a noun but long since is not a noun.

Word	Noun	Adjective	Verb	Adverb
lang syne	Yes	No	No	Yes
long since	No	No	No	Yes

As adverbs, lang syne and long since are synonyms defined as:

Other synonyms of lang syne include langsyne, long ago.

lang syne (noun)	long since (noun)
past times remembered with nostalgia

lang syne (adverb)	long since (adverb)
of the distant or comparatively distant past	of the distant or comparatively distant past