Using Matomo 3.5.1 (on Debian Stretch), I have downloaded the plugin from https://plugins.matomo.org/SiteMigration and unzipped it into the plugins directory.
When I run
./console migration:site --help from the main directory, I get this error message:
[InvalidArgumentException] There are no commands defined in the "migration" namespace.
Same happens when I try to migrate a siteID.
As version 1.0.8 has last been updated in 2015, I suspect it's not compatible anymore.
Correct. The plugin might not be compatible with Matomo 3+ as it isn't maintained anymore.