Is there an API for accessing historical Ubuntu Security Notices?

No, there is no API, but it should be fairly easy to create one by scraping the main CVE or partner CVE list into a db, instead of the USN website or email archive:

Here is some node code (tested with node 6.10.0) that will scrape the contents of https://usn.ubuntu.com/usn/ into a spreadsheet.

Here is what the spreadsheet will look like:

Here is the way to run the code:

var config = {};
config.searchIssuesAfter = new Date('2016-08-28');
config.reportName = 'Ubuntu Report';
config.securityNoticeURL = 'https://usn.ubuntu.com/usn/precise/';
config.usnBaseURL = 'https://usn.ubuntu.com';
config.numberOfPages = 6;
ubuntuReport = require('./getUbuntuData.js'); 
ubuntuReport.go(config);

Here is the supporting file getUbuntuData.js

var fs = require('fs');
var axios = require('axios');
var cheerio = require('cheerio');

module.exports = {
  go: function(config1) {
    //config
    var config = Object.assign({}, config1);
    var local = {};
    local.list = [];


    // Private functions  
    local.getPromiseUSNListFn = function(numPages){
      var promiseList = [];
      promiseList.push(axios.get(config.securityNoticeURL));
      for (var i = 2; i <= numPages; i++){
          promiseList.push(axios.get(config.securityNoticeURL+'?page='+i));
      }
      return promiseList;
    };

    local.sortByDateFn = function(a, b){
      return  b.timestamp - a.timestamp;
    };

    local.createSecurityNoticeListFn = function(response){
      var $ = cheerio.load(response.data);

      $('.notice').each( (i, elm) => {
        var securityNoticeNode = $(elm).find('h3 > a');
        var usn = securityNoticeNode.text().split(':')[0];
        var title = securityNoticeNode.text().split(': ')[1];
        var href = securityNoticeNode.attr("href");
        var dateNode = $(elm).find('h3');
        var dateStr = dateNode.contents().filter(function() {
          return this.nodeType == 3;
        }).text().trim();
        dateStr = dateStr.replace(/^\s*-\s*/,""); 
        var removeNewlinesRegEx = /(\r\n|\n|\r)/gm;
        dateStr = dateStr.replace(removeNewlinesRegEx,""); 
        dateStr = dateStr.replace(/(\d)(th|rd|nd|st)/,"$1");
        var date = new Date(dateStr);

        var infoObj = {
          usn: usn,
          title: title,
          href: config.usnBaseURL+href,
          timestamp: date.getTime(),
          date: dateStr
        };
        local.list.push(infoObj);
      });
    };

    local.getSecurityNoticeData = function(el){

        return axios.get(el.href).then( (response) => {
          el.summary = "";
          el.packages = [];
          el.versions = [];
          var $ = cheerio.load(response.data);
          console.log("Received : "+el.title);
          var removeNewlinesRegEx = /(\r\n|\n|\r)/gm;
          el.summary = $("h3:contains('Summary')").next().text().trim().replace(removeNewlinesRegEx," ");
          $("dl > dd").each((j, packageNode) => {
            el.packages.push($(packageNode).children().first().text().trim().replace(removeNewlinesRegEx," "));
            el.versions.push($(packageNode).children().first().next().text().trim().replace(removeNewlinesRegEx," "));
          });
        });
    };

    local.getPromiseListSecurityNoticesFn = function(){
      var promiseList = [];
      for (var i = 0; i < local.list.length; i++){
          promiseList.push(local.getSecurityNoticeData(local.list[i]));
      }
      return promiseList;
    };

    local.buildCSV = function(){
      var csv = "";
      csv += config.reportName+",,,,,,,,\n";
      csv += "Date,USN,Title,Package,Installed ?,Version,Urgency,Summary,Link\n";
      local.list.forEach((node) => {
        //node - final element
        var date = node.date;
        var usn = node.usn;
        var title = node.title;
        var packages = '"'+node.packages.join(", ")+'"';
        var installed = "?";
        var versions = '"'+node.versions.join(", ")+'"';
        var urgency = "?";
        var summary = '"'+node.summary+'"';
        var link = node.href;
        csv += date+",";
        csv += usn+",";
        csv += title+",";
        csv += packages+",";
        csv += installed+",";
        csv += versions+",";
        csv += urgency+",";
        csv += summary+",";
        csv += link;
        csv += "\n";
      });
      fs.writeFileSync(__dirname+"/../ubuntu_worksheet.csv", csv);
    };

    //Start 
    axios.all(local.getPromiseUSNListFn(config.numberOfPages))
      .then(axios.spread(function () {
        var i;
        for (i = 0; i < arguments.length; i++){
          console.log("Received page : "+i);
          local.createSecurityNoticeListFn(arguments[i]);
        }
        local.list.sort(local.sortByDateFn);
        for (i = 0; i < local.list.length; i++){
          if (local.list[i].timestamp < config.searchIssuesAfter.getTime()){
            //truncate list 
            local.list.length = i;
            break;
          }
        }
      }))
      .then(()=>{
        return axios.all(local.getPromiseListSecurityNoticesFn());
      }).then(()=>{
        local.buildCSV();
        console.log('data saved to ubuntu_worksheet.csv');
      });
   }
};