Terrain generation for loading old maps is completely broken!

I used to be a map creator back in 0.10.4-0.16 days. I would upload my creations to websites for people to download them and I loved what I did. I frequently like to redownload my maps and relive the memories. However: now, I can't redownload any of my old maps. When I try and join the worlds, the chunks in the ground where blocks were previously placed are all gone. This started since the 1.16 updates. I only just now discovered it, but I know I have been redownloading my old maps right before the 1.16.0 update. I don't know if this has to do with a new terrain generation implemented in recent versions, or if this is a bug. I put countless hours into my old creations and I still have every world file for them backed up on my computer. If I can't access them, all of it will be lost. If this is my new reality, is there any way to fix this and recover my worlds?

I have attached two worlds that I made years ago that both do not work now. There are also two screenshots of the NUK3TOWN world attached. The worlds linked are both flat seeds.

Thank you for the help!