From the Changelog:

The keys used are:
  !: modified feature, -: deleted feature, +: new feature

1.8.0 (2017-02-24):

  - $locale has been removed.  Mutt now respects the LC_TIME setting
    instead.  See also $attribution_locale.
  + $attribution_locale can be used to override the date formatting in
    attribution strings.  When unset, Mutt will use the locale
    environment, but note the default value of $date_format has a
    leading '!' which says to use the C-locale.
  ! Message-id and mail-followup-to headers are now preserved for recalled
    messages.
  + <unsidebar_whitelist> added to complement <sidebar_whitelist>.
  ! The pager position is reset to the top when toggling header-weed.
  ! IMAP messages moved to $trash via server-side copy are marked as read.
  + <root-message> jumps to the root message of a thread.
  ! Piped text attachments are charset converted.
  + Added %F to $attach_format, to show the content-disposition filename.
    %d will fall back to %F which will fall back to %f.
  + <rename-attachment> allows an attachment name to be changed, without
    modifying the underlying file's name.
  ! Mutt will look for the user's muttrc additionally in
    $XDG_CONFIG_HOME/mutt/.
  + Compressed mbox and mmdf files are now supported via open-hook,
    close-hook, and append-hook.  See contrib/sample.muttrc-compress
    for suggested settings.  Note this is a compile-time option:
    --enable-compressed.
  + When $flag_safe is set, flagged messages cannot be deleted.
  + The '@' pattern modifier can be used to limit matches to known aliases.
  + <mark-message> creates a hotkey binding to a specific message.  The hotkey
    prefix is specified via $mark_macro_prefix.
  + <setenv> and <unsetenv> can be used to add/remove environment variables
    passed to children.
  ! Mutt will now use the built-in OpenSSL SSL_set_verify() callback
    to verify certificates.  This allows better support for verifying
    chains, including alternative chain support.
  + $uncollapse_new controls whether a thread will be uncollapsed when a new
    message arrives.
  ! $to_chars and $status_chars now accept multibyte characters.
  + <subjectrx> allows replacing matching subjects with something else.
    This can be used to declutter subject lines in the index.
  + <edit-label> can be used to add, change, or delete a message's X-Label.
  ! Pattern expressions with ~y support label tab completion.
  + The header cache now also supports Kyoto Cabinet and LMDB as
    backend databases.