Here are some links to relevant documents:

• User:Kew Gardens 613/List of closed New York City Subway entrances — This is the most comprehensive list of closed subway entrances available, and includes entrance locations, the type of entrance closures, and when and why entrances were closed when available. This was made by Shaul Picker and Brian Lee.
• Reopening Closed Subway Entrances Using High Entry/Exit Turnstiles: Suggestions from the New York City Transit Riders Council (2001) — This report by the NYCTRC was the first to look into which closed subway entrances should be reopened and was the first to come up with a list of entrances to reopen.
• Re:Closed Subway Station Entrances (2015) — This is the MTA’s response to a FOIL request for a list of stations with closed subway entrances.
• WNYC Closed Entrances spreadsheet (2015) — This spreadsheet was provided to WNYC by the MTA in response to a FOIA request, and includes incomplete information on some closed entrances and staircases in the system.
• Review of the A and C Lines (2015) — This review of the A and C Lines by the MTA looked into places to reopen closed entrances. It recommended reopening entrances at 50th Street in the southbound direction, at 168th Street, and at Franklin Avenue if capital funding was available. It also recommended reopening entrances at Nostrand Avenue and Hoyt—Schermerhorn Streets if they became ADA-accessible.