Update comments

server/src/main/java/org/elasticsearch/cluster/ClusterFeatures.java

@@ -89,27 +89,15 @@ private Set<String> allNodeFeatures() {
 
     /**
      * Returns {@code true} if {@code node} can have assumed features.
-     * <p>
-     * This is true if it is of the next major version, or it is running on serverless.<br/>
-     * The next major version can be assumed because the next major version can only ever
-     * talk to the highest minor of the previous major, so any features added before that
-     * will always exist on the cluster.<br/>
-     * It can be assumed for serverless because we never go backwards on serverless,
-     * so once a feature is in the serverless environment it will always be there.
+     * @see {@link org.elasticsearch.env.BuildVersion#canRemoveAssumedFeatures}
      */
     public static boolean featuresCanBeAssumedForNode(DiscoveryNode node) {
         return node.getBuildVersion().canRemoveAssumedFeatures();
     }
 
     /**
      * Returns {@code true} if one or more nodes in {@code nodes} can have assumed features.
-     * <p>
-     * This is true if it is of the next major version, or it is running on serverless.<br/>
-     * The next major version can be assumed because the next major version can only ever
-     * talk to the highest minor of the previous major, so any features added before that point
-     * will always exist on the cluster.<br/>
-     * It can be assumed for serverless because we never go backwards on serverless,
-     * so once a feature is in the serverless environment it will always be there.
+     * @see {@link org.elasticsearch.env.BuildVersion#canRemoveAssumedFeatures}
      */
     public static boolean featuresCanBeAssumedForNodes(DiscoveryNodes nodes) {
         return nodes.getAllNodes().stream().anyMatch(n -> n.getBuildVersion().canRemoveAssumedFeatures());

server/src/main/java/org/elasticsearch/env/BuildVersion.java

@@ -38,7 +38,8 @@
 public abstract class BuildVersion implements ToXContentFragment, Writeable {
 
     /**
-     * Checks if this version can have features removed that are assumed in the currently running Elasticsearch.
+     * Checks if this version can operate properly in a cluster without features
+     * that are assumed in the currently running Elasticsearch.
      */
     public abstract boolean canRemoveAssumedFeatures();
 

server/src/main/java/org/elasticsearch/env/DefaultBuildVersion.java

@@ -49,6 +49,12 @@ final class DefaultBuildVersion extends BuildVersion {
 
     @Override
     public boolean canRemoveAssumedFeatures() {
+        /*
+         * We can remove assumed features if the node version is the next major version.
+         * This is because the next major version can only form a cluster with the
+         * latest minor version of the previous major, so any features introduced before that point
+         * (that are marked as assumed in the running code version) are automatically met by that version.
+         */
         return version.major == Version.CURRENT.major + 1;
     }