The last published version for Joomla! 3, Akeeba Backup 8.3.3, does not have that message. Which version of Akeeba Backup, Joomla, and PHP are you using?

Originally, I thought that you would see a message something something something 120 days which would immediately fail your backup. This did exist in some previous 8.x versions for the JSON API, but it was disabled in 8.3 knowing this is the last version branch for Joomla! 3. Hence my previous reply.

Now I get what you mean. You are seeing a backup warning, and the exit status of the CLI backup script is 1. Okay, this is normal and yes, we can change that by editing a file indeed.

There is indeed a warning issued reading “Your version of Akeeba Backup is more than 120 days old and most likely already out of date. Please check if a newer version is published and install it”. When you are using the command-line backup tool akeeba-backup.php the exit code is 1 when there are warnings in the backup. I assume you base your automation on the exit code being a clean 0 on a successful backup, therefore it makes sense to want to remove this warning. Yes, we can do that.

Edit the file administrator/components/com_akeeba/BackupPlatform/Joomla3x/Platform.php and find this block of code in lines 735-751:

		// If the release is older than 3 months, issue a warning
		if (defined('AKEEBA_DATE'))
		{
			$releaseDate = new Date(AKEEBA_DATE);

			if (time() - $releaseDate->toUnix() > 10368000)
			{
				if (!isset($ret['warnings']))
				{
					$ret['warnings'] = [];
					$ret['warnings'] = array_merge($ret['warnings'], [
						'Your version of Akeeba Backup is more than 120 days old and most likely already out of date. Please check if a newer version is published and install it.',
					]);
				}
			}

		}

Change the second line reading if (defined('AKEEBA_DATE')) to now read:

if (false)

Save the file. Now the code which was adding the backup warning is removed.

I would also like to ask you for your permission to set this ticket to public.

Thank you, I made this public. Let me know how it goes :)