Recent Changes Have Rendered the Documentation Incomplete
There should be a textual
protect link next to the
flag link at the bottom of each question, right under the question tags. However, I just noticed that the link doesn't appear for me on every question, despite the fact that the protection privilege page currently says:
Which questions can be protected?
Any question at least a day old can be protected and unprotected by users that have the privilege.
An answer from a few weeks ago over on SE Meta that indicates that there is a system-wide undocumented change:
15k users will still be able to protect but in order to do so the question must have at least one answer by a new user aka a user with < 10 rep on the site (which is the required rep limit to answer protected questions).
This is probably why many questions don't currently display the
protect link: because they haven't attracted any low-rep answers, and therefore shouldn't require protection. You might want to upvote this related question on SE Meta, or otherwise chime in there to indicate that this is clearly a bug in the current Stack Exchange documentation.