Change label text in Gutenberg’s built in excerpt panel

That label is a translatable text and its translation is retrieved using wp.i18n.__(), so you can change the text like you could in PHP via the gettext filter, except that in JS/Gutenberg, you’d use i18n.gettext.

Working example:

// Define our filter callback.
function myPluginGettextFilter( translation, text, domain ) {
    if ( text === 'Write an excerpt (optional)' ) {
        return 'Write an excerpt';
    }

    return translation;
}

// Adding the filter
wp.hooks.addFilter(
    'i18n.gettext',
    'my-plugin/override-write-an-excerpt-label',
    myPluginGettextFilter
);

And another way to change the text, without using hooks, is by using wp.i18n.setLocaleData() like so:

wp.i18n.setLocaleData({
    'Write an excerpt (optional)': [
        'Write an excerpt'
    ]
});
/* The format is:
wp.i18n.setLocaleData({
    '<original text>': [
        '<singular translation>',
        '<plural translation>'
    ]
});

// but just pass one translation (one array item) if there are no plural
// translation available, or if both singular and plural translations are
// identical
*/

And remember to load the script’s dependency, i.e. wp-hooks for the 1st example, and wp-i18n for the 2nd example.