The current version of the PT specification is 2.1. It is available, along with requests for modifications, at this page on Github.
Pluggable Transports are a concept developed by The Tor Project that provide ways to protect your data from aggressive censorship techniques, and can be included in a variety of tools. The Tor Project authored the original Pluggable Transports specification for use in the tor network environment.
In 2015 and 2016, a series of meetings of current transport developers and implementers circumvention tools met to determine broader, community requirements on what would be useful in a revised Pluggable Transports specification. These led to three proposals that were later merged into a draft specification for version 2.