Thank you for letting me know about the URL (https://jenkins.io/doc/pipeline/steps/p4/).  Jenkins created this based on the code, but I am not sure how they update it?  I agree it is confusing to read, I normally use the 'Pipeline Syntax' snippet generator and help bubbles to workout what options are available.  The older depotPath vs source should be backwards compatible.

For example 3 ways to define a stream on p4sync:

stream: '//streams/foo'
source: [$class: 'StreamSource', stream: '//streams/foo']
source: streamSource('//streams/foo')

I think the docs at the URL is generated from the same stuff as 'Pipeline syntax'. Basically there some options that are undocumented in 'Pipeline syntax' too. But the jenkins steps references could definitely be made more readable. Couldn't find a jira for that though... might file one tomorrow in jenkins-io.

Our previous job looked essentially like:

def p4Pin = 'now'
p4sync credential: 'cred', depotPath: env.P4_DEPOT, populate: [$class: 'ForceCleanImpl', quiet: true], pin: p4Pin

P4_DEPOT ="//depot/releases/3.4.x" was set in the job as a environment variable.
The depotPath had no ending '/..' before and after the upgrade this started failing due to p4-plugin generating the incorrect view client view:

View:
	//depot/releases/3.4.x //jenkins-somenodename-thejobname/3.4.x

I.e. it started mapping the wanted depot dir to a client sub dir instead of the root of the client sometime between 1.7.0 and 1.7.4.

This doesn't work for me at all with the latest p4 plugin. (jenkins version: 2.68 which is higher than the 2.6x mentioned above) All other plugins up to date.  I just get null.

I'm not using 'populate' like above though, i'm passing a workspace and spec:

def p4env = p4sync poll: true, credential: '',
 workspace: [$class: 'org.jenkinsci.plugins.p4.workspace.ManualWorkspaceImpl', charset: "", pinHost: false, name: client,
 spec: [$class: 'org.jenkinsci.plugins.p4.workspace.WorkspaceSpec', view: view]
 ]
echo "Sync p4changelist:"
 echo p4env.P4_CHANGELIST

also tried this as above:

p4env = p4sync poll:true, credential:'e6b4d2aa-df16-4ab5-aa9d-e8a149f1096f',
format: "<redacted>-${NODE_NAME}-${JOB_NAME}",
stream:'<mystream>',
populate:[$class:'AutoCleanImpl', delete:true, have:true, modtime:false, quiet:false, pin:'']

echo "Sync p4changelist:"
echo p4env.P4_CHANGELIST

results for both:
[Pipeline] echo
Sync p4changelist:
[Pipeline] echo
null

Code changed in jenkins
User: Paul Allen
Path:
src/main/java/org/jenkinsci/plugins/p4/build/P4EnvironmentContributor.java
http://jenkins-ci.org/commit/p4-plugin/65279c9d1158fa247e406d6146ef8395d39906c6
Log:
Copy environment map to EnvVars.

JENKINS-37584

You can download the latest HPI build here: https://ci.jenkins.io/job/Plugins/job/p4-plugin/job/master/100/