How to retrieve "Internal reason for ban" field

I don't think that there's an API that will return you all the banned users, only the ban details for a specific user. 

When you query the users, you can grab the "banned" and see if it's true. If it's true, you can use the API v1 to see the details. Something like this:

<#assign banData = restadmin("/users/id/${item.id}/moderation/banned/details").bans.ban>
<#list banData as ban>
  Banned by: <a href="/t5/user/viewprofilepage/user-id/${ban.banned_by}" target="_blank">${users.firstNameLastInitial(ban.banned_by)}</a><br>
  Ban reason: <#if ban.banned_reason == "">empty<#else>${ban.banned_reason}</#if><br>
  Public ban reason: <#if ban.public_banned_reason == "">empty<#else>${ban.public_banned_reason}</#if><br>
  Banned on: ${ban.date_start?number?number_to_datetime}<br>
  Banned until: <#if ban.date_end == "-1"><b>forever</b><#else>${ban.date_end?number?number_to_datetime}</#if>
</#list>