public class Config implements WebMvcConfigurer

WebMvcConfigurer is not deprecated.

I produced the usable code following your question:

import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.servlet.config.annotation.CorsRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;

@Configuration
public class Config implements WebMvcConfigurer {

    @Value("${allowed-origins:*}")
    String allowedOriginsPattern;
        // contains "http://localhost:8080,http://127.0.0.1:8080,http://[::1]:8080"

    @Override
    public void addCorsMappings(CorsRegistry registry) {
       String[] origins = allowedOriginsPattern.split(",");

       registry.addMapping("/**")
           .allowedOriginPatterns(origins)
           .allowedMethods("GET", "OPTIONS", "POST")
           .allowCredentials(true);
    }
}

In the application.yml, I added:

server:
  servlet:
    context-path: /

allowed-origins: "http://localhost:8080,http://127.0.0.1:8080,http://[::1]:8080"

An incorrect context-path could be a reason for a CORS problem.

The example repository was here.

To try, after starting Spring Boot, please visit in your browser:

http://localhost:8080
http://127.0.0.1:8080
http://[::1]:8080

Example output in the browser:

CORS
GET http://127.0.0.1:8080/api
GET request page
POST http://127.0.0.1:8080/api
POST request page
GET http://localhost:8080/api
GET request page
POST http://localhost:8080/api
POST request page
GET http://[::1]:8080/api
GET request page
POST http://[::1]:8080/api
POST request page

In the example, the JavaScript I used:

<script>
                async function getter(uri, elementId){
                    const responseGet = fetch(uri, { "method": "GET" })
                            .then(response => {
                                if (!response.ok) {
                                    return response.text()
                                        .catch(() => {
                                            throw new Error(response.status);
                                        })
                                        .then(({message}) => {
                                            console.log(message);
                                            throw new Error(message || response.status);
                                        });
                                }
                            return response.text();
                            });

                            
                        const responseGetText = await Promise.resolve(responseGet);
                        

                        const elementGet = document.getElementById(elementId);
                        elementGet.innerHTML = elementGet.innerHTML + "<div>" + responseGetText + "</div>"

                }
                
                async function poster(uri, elementId, body){
                    const responsePost = fetch(uri, { "method": "POST", body: body })
                        .then(response => {
                            if (!response.ok) {
                                return response.text()
                                    .catch(() => {
                                        throw new Error(response.status);
                                    })
                                    .then(({message}) => {
                                        console.log(message);
                                        throw new Error(message || response.status);
                                    });
                            }
                        return response.text();
                        });

                        
                    const responsePostText = await Promise.resolve(responsePost);
                    

                    const elementPost = document.getElementById(elementId);
                    elementPost.innerHTML = elementPost.innerHTML + "<div>" + responsePostText + "</div>"
                }

                async function main(){
                    

                   await getter("http://127.0.0.1:8080/api", "getipv4");
                   await poster("http://127.0.0.1:8080/api", "postipv4", JSON.stringify({"user": "user"}));

                   
                   await getter("http://localhost:8080/api", "getlocalhost");
                   await poster("http://localhost:8080/api", "postlocalhost", JSON.stringify({"user": "user"}));

                   
                   await getter("http://[::1]:8080/api", "getipv6");
                   await poster("http://[::1]:8080/api", "postipv6", JSON.stringify({"user": "user"}));


                }
                main();
                </script>