Is The Warriors an accurate depiction of gang relations in 1970s New York City?

No, it was not.

New York City gangs, like most other street gangs in the United States, are almost exclusively broken down along racial and ethnic lines. The Warriors themselves were a multicultural group, which, given the racism present in New York City in the 1970s is unlikely to have formed organically. Even in present day America, street crime organization safe almost always race/ethnic or neighborhood based, precluding the formation of mixed member gangs.

Females "gangs" as depicted in the film have almost never existed. When females do act as criminals they usually do so alone, as part of a male/female partnership or as part of a small group of non-violent females. Even modern female street and prison gangs are far smaller, less cohesive and far less violent than their male counterparts.

Finally, a "gang truce" would have attracted the attention of the police as that many criminals in a single location would have alerted the police to their presence prior to any such event occurring. Coupled with the fact that most criminal organization have serious issues with informants, any attempt for any group larger in number than the leadership caste of the various gangs meeting would have been a near impossibility without attracting numerous law enforcement organizations for what would d have been mass arrests.

The Warriors is decent entertainment, but only that: ENTERTAINMENT

References:

http://www.thefader.com/2005/10/03/new-york-mythology/

http://www.dc.state.fl.us/pub/gangs/prison.html

http://www.businessinsider.com/most-dangerous-prison-gangs-in-the-us-2013-4