There is currently no mechanism in the Open Problem Library to track what version of PG the problem was authored against. At best the community tries to ensure that all problems in the OPL work with the current version of WeBWorK, and update them if they break.
If you are writing new problems in the most recent version of WeBWorK, then clicking on the "Sample Problems" button in the problem editor will take you a collection of problems demonstrating various techniques that use the most recent problem standards. In the upcoming release (2.20) you will be able to load these sample problems directly into the Problem Editor.
As WeBWorK instances are self-hosted, there is no central repository of assignments. The best you could hope for is someone who reads these forums being willing to share their assignments with you.