Google app script to remove old forum messages. Paste into https://script.google.com and schedule or whatever.

clean_emails.js

function cleanUp() {
 
  // I think this refers to the date when the thread was started.
  var delayDays = 90;
  
  var max = 200;
  var offset = 0;
  var query = 'category:forums older_than:' + delayDays + 'd'
  
  while (true) {
    var threads = GmailApp.search(query, offset, max);
    Logger.log("fetched %s", offset + threads.length);
    
    for (var i = 0; i < threads.length; i++) {
      Logger.log("removing %s", offset + i);
      threads[i].moveToTrash();
    }
    if (threads.length < max) {
      break;
    }
    offset += max;
  }
  
}