It will be easier for you to initially not record a notice/message containing certain words than to later search for and delete this saved string in the window.

Try using this script code instead of your script:
Code
on *:NOTICE:*:*:{
  if (*try this new feature* iswm $1-) { return }
  if (!$window(@Notice)) window @Notice
  aline -p @Notice $timestamp $1-
}

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