Understood.

writing the command into a command.groovy and then piping it into the groovy CLI command worked

This pipeline

writeFile file: 'cmd.groovy', text: getCommandText()
def getCommandText() {
'''
println 'HelloWorld'
'''
}
sh 'cat cmd.groovy | java -jar ./jenkins-cli.jar -ssh -user * -i * -s http://serv:14/ groovy ='

looks like this

Running shell script
+ cat cmd.groovy | java -jar ./jenkins-cli.jar -ssh -user **** -i **** -s http://serv:14/ groovy =
HelloWorld

Thanks for clarifying

When trying to execute a groovy command over the ssh method, Jenkins complains about me wanting to use the -remoting mode

Only the remoting mode can access local files.

which mentions that some commands wouldn't work, but not which ones

Basically a direct consequence of extensibility, there can never be a complete list.

Same when I try groovy from standard-in

That's not stdin. stdin is when you use a pipe or input redirection. What you're doing isn't supported by the documentation at all (you're telling it to read from a file called "println"):

java -jar jenkins-cli.jar -s … groovy [SCRIPT] [ARGUMENTS ...]
SCRIPT : Script to be executed. File, URL or '=' to represent stdin.

Same when I try groovy from standard-in
java -jar ./jenkins-cli.jar -ssh -user **** -i **** -s http://myserver:1234/jenkins/testing/ groovy println 'hello world'ERROR: This command is requesting the deprecated -remoting mode. See https://jenkins.io/redirect/cli-command-requires-channel

This doesn't seem to work.

When trying to execute a groovy command over the ssh method, Jenkins complains about me wanting to use the -remoting mode:

java -jar ./jenkins-cli.jar -ssh -user **** -i **** -s http://myserver:1234/jenkins/testing/ groovy command.groovy 
ERROR: This command is requesting the deprecated -remoting mode. See https://jenkins.io/redirect/cli-command-requires-channel

The URL btw points to the not yet quite complete CLI documentation (which mentions that some commands wouldn't work, but not which ones...)

The command was a simple println

Fixed in 2.46

Code changed in jenkins
User: Daniel Beck
Path:
core/src/main/java/hudson/cli/GroovyCommand.java
http://jenkins-ci.org/commit/jenkins/0d3d6b65bf134ba01d67908db63212dc54aa1c58
Log:
[FIX JENKINS-41765] Allow groovy CLI command via SSH