They implicitly announced it, when they posted two "coming to SteamOS" ads in 2014 and 2015 respectively. SteamOS is Linux (based on Debian). It was before GOG added Linux support, so I assume that's why they didn't have such ads on GOG, but those ads were legit Steam ads, not some fake, and one of them said explicitly "pre-order now to save 10%".
They confirmed multiple times through e-mail to different people, that they were working on the Linux version. It wasn't actual CDPR studio, but they outsourced it to Virtual Programming, who also ported TW2 to Linux. Things were going well until 2016, when the tone of their answers started indicating some problems and eventually turned to complete "no plans". They didn't give a reason, but some said (not sure how valid), that it could be related to some shareholders who weren't Linux friendly, and one former Virtual Programming employee said it could be related to Steam Machines console project not growing fast enough. Either way, CDPR / VP didn't release anything in the end, and the project now can be considered scraped.
In practice though, situation today isn't the same as it was then. Such porting is cheaper, there are better tools like Vulkan and Linux gaming market (PC, not consoles) has grown since then too, so there is some hope that CDPR can do better today and avoid the above problems with Linux release.