gtkbuilder.rnc says:

object = element object {
  attribute id { xsd:ID } ?,
  attribute class { text },
  attribute type-func { text } ?,
  attribute constructor { text } ?,
  (property | signal | child | ANY) *
}

However, the parser uses G_MARKUP_COLLECT_OPTIONAL when looking for the "class" attribute.

Apart from the aesthetics of it, enforcing "class" as a mandatory attribute for "object":s is a nice idea because it can help Vala [1]. For GTK+ template classes, the Vala compiler ensures that all [GtkChild] class members in Vala have a matching XML object with the right type. However, it doesn't know about "type-func". So, if only the "type-func" attribute is used, then it will fail to compile because it won't know about the XML object. See vala/codegen/valagtkmodule.vala

It seems easier to fix this in GtkBuilder than to teach valac to get the type from the "type-func". But I know little about Vala, so I might be wrong.

Since this will break existing run-time behaviour, it can only be done in master, and not fit for gtk3. Although, note that "class" attributes are vastly more common than "type-func", so breakage should be minimum.

[1] https://bugs.webkit.org/show_bug.cgi?id=175937#c16