Historical data on world's forest area
When I need data about past and future projected land use change, I usually refer to the global dataset that was developed for the global climate simulations CMIP6. 
The dataset is the Land Use Harmonization V.2 LUHv2 and it was developed by researchers at the University of Maryland. 

Reading the supporting document of the dataset, among others, the following characteristics are reported:


  
  global domain;
  850­‐2100 annual land-use states, transitions, and gridded mgt layers;
  0.25 x 0.25 degree spatial resolution;
  12 possible land-­use states including separation of Primary and Secondary natural  vegetation into Forest and Non‐forest sub-­types,
  Pasture into Managed Pasture and  Rangeland, and Cropland into
  multiple crop functional types
  


The dataset is composed of a set of simulations using different land surface models:


  These datasets are being developed as a contribution of the Land-Use
  Model Intercomparison Project (LUMIP) to the Forcing Group for CMIP6


Data can be downloaded from this page.

Of course, the layers you find here are "just" models' results, but as much as past observations made a good calibration possible, the variations reported are likely be the most accurate information about the evolution of the world forested areas currently available.