The Nintendo Wiki has a simple spoiler policy. We simply have spoilers. A few of the spoilers are marked, but almost all are not. For a wiki to have full coverage of any topic it must have spoilers, and marking every single spoiler would be difficult and it would disrupt the flow of the articles.
So, it is recommended that if you do not want too see spoilers that you do not read articles related to the topic that you do not want spoiled. It is especially recommended that you do not read character pages if you do not want to see spoilers about them, since they generally tend to have spoilers on them.