Deprecate respond-to mixin, Zen uses css @media directly instead.

assets/sass/components/flex-group/_flex-group.scss

@@ -16,7 +16,7 @@
     flex-grow: 1;
   }
 
-  @include respond-to(s) {
+  @media (min-width: 666px) {
     &--2 > * {
       @include flex(2, $gap);
     }

assets/sass/components/header/_header.scss

@@ -8,7 +8,7 @@
   gap: var(--gutters);
   background-color: var(--color-header-bg);
 
-  @include respond-to(s) {
+  @media (min-width: 666px) {
     flex-direction: row;
   }
 
@@ -44,7 +44,7 @@
 
   // Wrapper for any blocks placed in the header region.
   &__region {
-    @include respond-to(s) {
+    @media (min-width: 666px) {
       margin-inline-start: auto;
     }
   }

assets/sass/components/tables/_tables.scss

@@ -2,7 +2,7 @@
 %zebra-table {
   --table-cell-padding: .5rem;
 
-  @include respond-to-max(s) {
+  @media (max-width: 666px) {
     --table-cell-padding: .3rem;
   }
 
@@ -34,7 +34,7 @@
 %responsive-table {
   --table-gap: .5rem;
 
-  @include respond-to-max(s) {
+  @media (max-width: 666px) {
     th {
       display: none;
     }