Message about customized worlds support keeps appearing upon load

I have a map I've been working on for 5 or 6 years that I've been updating to the latest version of Minecraft as those versions have been released along the years. In the past, I've gotten the message stating "Customized worlds are no longer supported." Last time I recalled it happening, I just clicked any button to continue and didn't worry about it. The message never came up again. But since the 1.16 update (and now 1.16.1) this message keeps popping up every time I try to load the map. I don't know why this is happening. I've tried the Optimize World option but that didn't make a difference.