Is it illegal to download cracked version of the previous Adobe Softwares like CC 18?
Downloading and using of cracked software is definitely illegal !!
Yes, still illegal. All cracked software is illegal. Always.
FYI, all versions, including older, superseded ones, are always owned by Adobe at all times.
They may be unsupported and not available for download from Adobe - but they're still the property of Adobe.
Copyright (owning) lasts about 100 years, and no law says they have to sell what they own.