The current code always does past two releases, unless you want it to do three, I guess:

                majorReleases, err := b.getMajorReleases(ctx)
                if err != nil {
                        return err
                }
                var selectedReleases []string
                for _, r := range majorReleases {
                        if strings.Contains(backportComment.Body, r) {
                                selectedReleases = append(selectedReleases, r)
                        }
                }
                if len(selectedReleases) == 0 {
                        // Only backport to major releases unless explicitly
                        // asked to backport to the upcoming release.
                        selectedReleases = majorReleases[:len(majorReleases)-1]
                }

So perhaps the feature request here is to add a way to only backport to the most recent.

/cc @cagedmantis Another issue you could look at.

the bot opened up issues to Go1.12 and Go1.13

A number like "1.13" is enough. There isn't a need for a space in front or after. This is because getMajorReleases returns strings like []string{"1.9", "1.10", "1.11"}.

The reason gopherbot opened both issues is because it looks in the entire comment body, and you had mentioned 1.12 in the paragraphs above, before asking gopherbot.

Improving that is tracked in issue #27489.

which then caused another set of backporting issues to be opened

This is a separate issue and is tracked in #32533.

I'll close this because all of the problems described here are covered by more specific existing issues.