The devil is in the detail as they say. Pixbay is verry waffly on what is and is not permitted[1], and in their "conclusion" they basically wash their hands of the risk, putting the onus back on the user: Understand that Pixabay images can be used in many instances without asking permission from the image author. However, it is your responsibility, to make sure the depicted content (persons, logos, designs, property,…
(Disclosure: I work for Canva, the parent company of Pixabay). Pixabay does offer a legally binding license to you, written here: https://pixabay.com/service/terms/#license > Under the Pixabay License you are granted an irrevocable, worldwide, non-exclusive and royalty free right to use, download, copy, modify or adapt the Content for commercial or non-commercial purposes. Attribution of the photographer, videographe…
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a. Sale or distribution of Content as digital Content or as digital wallpapers (such as on stock media websites or as NFTs);
b. Sale or distribution of Content e.g. as a posters, digital prints, music files or physical products, without adding any additional elements or otherwise adding value
c. Depiction of identifiable persons in an offensive, pornographic, obscene, immoral, defamatory or libelous way; or
d. Any suggestion that there is an endorsement of products and services by depicted persons, brands, vocalists and organisations, unless permission was granted.
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This puts is squarely in the 'non-libre' or 'non-open' side of things.
For example, this would be in violation of any GPL-like license. While software licenses don't necessarily apply to things like pictures, I would be hesitant to include any pictures from Pixabay in any libre software I was creating.