For completeness and in case it helps someone else in the future.
This CSS that I tried to use to hide some custom user fields:
/* Hide the social media platforms from the lower part of their public profile page */
.instagram, .tik-tok, .facebook, .you-tube, .x-\(twitter\), .vimeo {
display: none;
}
Needed to become this to work across desktop and mobile:
/* Hide the social media platforms from the lower part of their profile page */
.user-main .about .details .public-user-fields .you-tube, .user-main .about .details .public-user-fields .vimeo, .user-main .about .details .public-user-fields .instagram, .user-main .about .details .public-user-fields .x-\(twitter\), .user-main .about .details .public-user-fields .facebook, .user-main .about .details .public-user-fields .tik-tok {
display: none;
}