I mean it in several senses.

In the labeling sense, it's a warning label, in that it serves to tell people who may wish to avoid something that it is present in the product, much like warnings saying "may contain tree nuts" on products that are potentially cross-contaminated. (As compared to a label that people are likely to seek out, like "100% juice", which is regulated differently to prevent people from using it when it doesn't apply.)

In the computing sense, it's a warning, in that it doesn't stop you from ignoring it if you want to, but some people may wish to `-Werror` / `-D warnings`.