Ok so today I was trying to redownload and reinstall some CTs and CTRs that I've deleted until I discovered that the RealTycoon3 website was shut down because it had expired. So this was quite upsetting. I literally went on a research spree to see if any downloads by RealTycoon3 was still available. Sadly, most, but not all, were gone; some were retrieved by a Brazilian website through MediaFire, which luckily I was able to download and install some CTs onto RCT3. Anyways, now that realtycoon3.com is completely gone. I don't really know which website to go onto get more CTs other than, shyguysworld and the Custom Scenery Depot.

So many game sites have died lately, usually because the traffic no longer justifies the expense of domain renewal and server fees. And many individual sites get purchased by holding companies who will shut them down immediately if they do not generate the advertising revenue expected, regardless of popularity or daily traffic (RIP NoGripRacing).

Best course of action is probably to put the stuff you OWN (not just possess) onto Archive.org. There's a lot that got archived (a lot more than you'd expect) for the early RCT series. I've scrounged around and found that RCTgo is one of the few remaining havens for RCT3. (NEDesigns is the best for pre-RCT3 and all things OpenRCT2.)
Or you could be like some banworthy people who upload other users' work without credit.
I'm pretty bad about not keeping copies of ALL the originals, but I have compressed files containing most of the +12GB of custom content I've downloaded and imported (with still more yet to import!).

I've long been ridiculed for my habit of backing up mods I download for any games; "why bother? you can always download the file again". Vindication comes when reading posts from people seeking those files after the original host sites have disappeared. Recently "wasting time" cataloging and backing up, to flashdrive and cloud, those old disks (a few 20+ yr old CDs are no longer readable).