> The warning disappears when we use tm_tag_local_var_t for "Variables" member in group_MESON. Goto still works this way but I'm not sure if this is a valid "fix" or this might have other side effects?
Yes, this is a valid fix, it has to appear in some group (in fact, it could be added to any of the groups, since local variables aren't displayed, it doesn't matter).
I added this warning to Geany so we don't forget to update groups when new kinds are introduced in which case symbols wouldn't be displayed in the sidebar.