That config option is removed in 0.55. Read the “Notes” section in the release notes. In short, you need to do something ala:
{{- $pages := .Data.Pages -}}
{{- $limit := .Site.Config.Services.RSS.Limit -}}
{{- if ge $limit 1 -}}
{{- $pages = $pages | first $limit -}}
{{- end -}}
In your template.
And for people asking “why on earth do you need to add these breaking changes”:
We used to have lots of special code paths and a different rendering implementation for some of the output formats (e.g. RSS). Now we have more or less 1 simple function. Which means less code to maintain and test.