I would have ignored that, too.
I have been guilty of ignoring those types of letters myself. My Explorer, which we just recently got rid of, hadn't run for 4 years. So when it came time to renew my tags for my plates, I didn't do it because I wasn't driving it. It was in our driveway, and you wouldn't know it wasn't running by looking at it, it was in great cosmetic shape. But I got a letter calling it an eyesore since it didn't have current tags. Sitting on the street, I maybe could have understood that. But it was in our driveway, on our property. So I had to keep current tags on it, which also meant I had to keep insurance on it, because you have to show proof of insurance when renewing tags. That is a stupid ordinance. (In my opinion.
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But, if my Explorer, which looked like a perfectly good, running vehicle, even if it wasn't, was considered an "eyesore", what is all the crap in their yard? They probably are ignoring letters, too. But I think trash in the yard is definitely worse than having out of date tags. They can't even pull all the way into their driveway, for all the stuff!