Many capital-I Informative RFCs reserve things like HTTP Header Names. Some of the easiest examples are April Fools RFCs that reserve all kinds of things, and while some people do like to use HTTP 418 I'm a Teapot (https://http.cat/418), that's not a capital-S Standard.
The IETF has workflows for Informative things versus Standard things. It's still useful to "reserve" things in Informative RFCs in case the RFC later moves to the Standards Track. Or also as some of the April Fools RFCs seem to indicate the internet takes its jokes pretty seriously and you never know when an Informative thing maybe adopted just because it was interesting to some developer for some project.
The IETF has workflows for Informative things versus Standard things. It's still useful to "reserve" things in Informative RFCs in case the RFC later moves to the Standards Track. Or also as some of the April Fools RFCs seem to indicate the internet takes its jokes pretty seriously and you never know when an Informative thing maybe adopted just because it was interesting to some developer for some project.