You will lose your user settings, but here's some CSS to ensure that the summaries show on mobile too even when logged out. The reason I then hide the new user settings is because this CSS forces the summary to be there no matter what on mobile, essentially breaking the user settings (well, the mobile setting but I hide both new settings because why have one and not the other...).
If Flarum had something like a logged-out class in the body tag then this could be done without breaking the user settings.
@media (max-width: 767px) {
.DiscussionListItem-info .item-excerpt {
display: block !important;
}}
.Settings-Summaries {
display: none;
}
