does freemarker support show all variable in data-model?

An even more detailed way would be this macro:

<#macro dump_object object debug=false>
    <#compress>
        <#if object??>
            <#attempt>
                <#if object?is_node>
                    <#if object?node_type == "text">${object?html}
                    <#else>&lt;${object?node_name}<#if object?node_type=="element" && object.@@?has_content><#list object.@@ as attr>
                        ${attr?node_name}="${attr?html}"</#list></#if>&gt;
                        <#if object?children?has_content><#list object?children as item>
                            <@dump_object object=item/></#list><#else>${object}</#if> &lt;/${object?node_name}&gt;</#if>
                <#elseif object?is_method>
                    #method
                <#elseif object?is_sequence>
                        [<#list object as item><@dump_object object=item/><#if !item?is_last>, </#if></#list>]
                <#elseif object?is_hash_ex>
                        {<#list object as key, item>${key?html}=<@dump_object object=item/><#if !item?is_last>, </#if></#list>}
                <#else>
                    "${object?string?html}"
                </#if>
            <#recover>
                <#if !debug><!-- </#if>LOG: Could not parse object <#if debug><pre>${.error}</pre><#else>--></#if>
            </#attempt>
        <#else>
            null
        </#if>
    </#compress>
</#macro>

<@dump_object object=.data_model/>

This gives you a full dump of your data model.

There's no universal solution possible for that, but you can try

<#list .data_model?keys as key>
  ${key}
</#list>

This works if the data-model is just a usual Map or JavaBean, but for more sophisticated data-models it's up to the data-model implementation if it supports ?keys and if it indeed returns everything.

You also have the variables that you set in the templates, which can be listed like above, only instead of .data_model use .globals, .namespace (which means the current template namespace) and .locals.

You may also have Configuration-level shared variables, and there's no way to list those purely from FTL (you could write a custom TemplateMethodModel for it that reads Configuration.getSharedVariableNames() though, and call it from the template).

Of course, ideally, FreeMarker should have a <#show_variables> directive or something, that does a best effort to show all this... but sadly there is no such thing yet.