Right now, secondary link icons are loaded using modules/styles/mw.echo.icons.less, which contains manually written rules like:

.oo-ui-icon-thanks {
        .background-image('../icons/thanks-ltr.svg');
}

Since these are normal OOUI icons (24x24px and everything), it would be nicer and relatively easy to put them into a ResourceLoaderImageModule similar to the emailicons module, by generalizing ResourceLoaderEchoImageModule. That would also allow extensions to define their own secondary link icons, which we want to do in Thanks: T151768: Move Thanks icons to Thanks repo