As recently as 6 or 7 years ago I maintained some apps that forced 5.5 compatibility mode. Because they were poorly architected in a shitty framework and no one was willing to do or pay for or train for a rewritten version. They were finally migrating to .NET when I left. It was the govt so they are likely wrapping up that migration now.
Have you considered just forcing everyone to access your sites via Internet Explorer 5.5?
Ahh the old Nutscrape Aggrivator. Refused to update with Microsoft standards and spent the next 8 years being a pain in the ass for website compatibility.