Yes—community members reported that the older free Nik Collection could still be downloaded from official or reputable sources, including a direct DxO page and links referenced by Photo Rumors. If you can get it from DxO or another clearly legitimate publisher, that is far safer than using random download sites.

Avoid unofficial file-hosting sites if possible. Even if copies are available there, they may be legally questionable and potentially unsafe.

As for Photoshop CC 2018 compatibility: older plug-ins sometimes continue to work with newer Photoshop versions, but support is not guaranteed. Because this is an older, discontinued free release, expect possible compatibility issues or limited support with CC 2018. The safest approach is to use an official installer and test it on your system.