According to https://www.geany.org/manual/reference/legacy.html it's deprecated since Geany 1.26, don't know the API version.
I was asking because if the API version used by the plugin was older, you'd have to bump the API too. But I've just checked and 226 which the plugin uses corresponds to the 1.26 release so it's OK for this plugin. But you should check other plugins too.
As I said, the patch looks good to me and I have no problem with it - I was just wondering about the reason and you gave me the answer.