From my experience it is not needed at all. All my rule files are working without extra init files.
It is not necessary to work but better for troubleshooting and testing.
If you look inside the extensions in the Checkmk github then all are using the type declaration.
But this was existing since some versions i think. (in 2.1 i saw the first migrate functions)
And i ignored it all the time 
This was also existing before. But it was not so clear in the older versions.
_(“String”) meant there this string can be replaced with translation strings.