PS, if you want to also remove them from sites that already have them downloaded, then you would need to use not the post-root-install-package callback, but also the post-update-cmd. So in that case, rather than the code in the Github PR, the Scripts section of your composer.json would look something like this
"scripts": {
"post-root-package-install": [
"php -r \"copy('.env.example', '.env');\"",
"composer run remove-old-wp-themes"
],
"post-update-cmd": [
"composer run remove-old-wp-themes"
],
"remove-old-wp-themes": [
"rm -rf web/wp/wp-content/themes/twentyten",
"rm -rf web/wp/wp-content/themes/twentyeleven",
"rm -rf web/wp/wp-content/themes/twentytwelve",
"rm -rf web/wp/wp-content/themes/twentythirteen",
"rm -rf web/wp/wp-content/themes/twentyfourteen",
"rm -rf web/wp/wp-content/themes/twentyfifteen",
"rm -rf web/wp/wp-content/themes/twentysixteen",
"rm -rf web/wp/wp-content/themes/twentyseventeen",
"rm -rf web/wp/wp-content/themes/twentynineteen",
"rm -rf web/wp/wp-content/themes/twentytwenty"
],
"test": [
"phpcs"
]
}