Get all data from an external and paginated Swagger API using Java / Feign / Spring
I see these lines in your code :
List&lt;Item&gt; items = restPage.getContent();
List&lt;Item&gt; result = new ArrayList&lt;Item&gt;();
result.addAll(items);

You can use the ArrayList(Collection&lt;? extends E&gt; c) constructor obtaining the same result:
List&lt;Item&gt; result = new ArrayList&lt;&gt;(restPage.getContent());

Your filtering code looks good to me and for the for loop adding elements to an existing list for me you made the right choice. Although it is possible to add new elements to an existing list using streams this is discouraged because easy cause of errors, see for example this thread.