prope is not included in release 1.21.1

[indiejames]

Attempting to run

go get github.com/zyedidia/generic/prope

Results in the following error:

go: module github.com/zyedidia/generic@upgrade found (v1.2.1), but does not contain package github.com/zyedidia/generic/prope

I can retrieve the entire package with

go get github.com/zyedidia/generic

But when I try to use propfe with
package main

import (
"github.com/zyedidia/generic/prope"
)

func main() {
data := "=ℇ∞∏"
runes := []rune(data)
r := prope.Newrune
r.Insert(1, []rune{32})
}

I get the error message

main.go:7:2: no required module provides package github.com/zyedidia/generic/prope; to add it:
go get github.com/zyedidia/generic/prope

If I download the 1.21.1 release directly from GitHub I can see that the distribution archive does not contain the `prope` package.