The change, and the bug behind it
Admin post list tables in WordPress have had their row semantics backwards since the markup was written. Screen readers announce each row using the checkbox column header, so a user hears "Select All" where the post title should be. WordPress 7.1 fixes it.
The repair is structural. The checkbox column moves from a th to a td, the post-title column becomes a th carrying scope="row" plus an aria-label holding the title, and collapsed cells in the responsive view switch to a flex layout.
That is also why it breaks things. Plugins using CSS or JavaScript selectors written against specific th or td elements in those tables were relying on the arrangement that is being corrected. Search Engine Journal reports public-facing site functionality should be unaffected, which keeps this inside the admin.
What to do about it
Read the changelogs before the update, not after
If WordPress runs your blog or content layer, check whether the plugins that customize admin list screens have declared 7.1 compatibility before you take the update. The failure mode is a broken admin column or a dead bulk action rather than a down site, which means it will be found by whoever next tries to publish rather than by monitoring.