Documentation for older ThinLinc versions can be downloaded here as part of the client bundle.

ThinLinc Documentation and Clients in Zip format
ThinLinc Client Version Release date  
ThinLinc Client 4.8.1 2018-05-08 Download
ThinLinc Client 4.8.0 2017-12-18 Download
ThinLinc Client 4.7.0 2016-11-02 Download
ThinLinc Client 4.6.0 2016-06-15 Download