If someone uses the setting in the module but is running PuppetDB 1.0.x, I'm about 99% sure that it would just silently ignore the setting.  I'm pretty darn sure that our config parsing does not currently check for extra/unsupported key/val pairs in the config files (though I would prefer if it did).

We could certainly use semantic versioning and deprecation rules, and that might be the best way to go... but it's going to cause a bunch of spaghetti code in here that makes me tempted to just document the compatibility constraints and not hassle with it.  The former would probably be a better end user experience but I'm not 100% sure how much payoff there would be.

And yes, we'd have to add a fact or something that would detect what version of PuppetDB was installed (if any).  Actually, I don't know if it could be a fact because then it wouldn't work on the first run (before they'd installed the package initially)... unless we had some if/pick statements that were smart enough to distinguish between the installed version (or lack thereof), the latest version available (if they're using 'present'), or the user-specified version (if they pass the param).