I am trying to set Azure DevOps to skip a stage if a message does not
start with a given text.
If I am not misunderstand, the condition you want is if the message match the start with maven-release-plugin
, the current stage will be queued.
If this, the condition you write is not correct, I think you should specified it:
startsWith(variables['Build.SourceVersionMessage'], '[maven-release-plugin]')
As I tested on my pipeline:

And in fact, the value of this variable is Deleted 121321. Here is the result:

As you can see, that it is successful to skip the stage. My logic is, the value of Build.SourceVersionMessage
should start with othermessage
. But in fact, in my pipeline, it's value is Deleted 121321
. Not match, so skip this stage.
(Delete 121321 is not only my PR name, I just set the commit message as the default PR name.)
Updated 2:
Though my test logic is not incorrect, but after I reproduced with YAML and many other method tested, such as use Build.SourceVersion which can only got after the source pulled.
Yes, you are right about that Build.SourceVersionMessage does not has value in Job level. As my tested, it indeed null in job level.
But, unfortunately, this is not a bug. This is as designed in fact.
We can think that the source repos be pulled locally only the stage job begin to execute, right? You can see the Checkout log, it record the process about pull source file down.
If the stage does not be executed, the source will not be pulled down. But also, if no source pulled, the server will also could not get the value of Build.SourceVersionMessage because of no source history. That's why I also tested with the variable Build.SourceVersion in the Job level.

We can not use these two variable at the agent job level because it hasn't pulled sources yet so Build.SourceVersionMessage is null. You'll need to copy it to each step in your pipeline. This is what confirmed by our Product Group team.
But, I still need to say sorry. Sorry about our doc is not too clear to announce that this could not used in agent job level.