What happens when a Spring Boot 3 / Spring 6 application is upgraded to Spring Boot 4 / Spring 7 without addressing RestTemplate and other deprecations?

The application still compiles and runs - RestTemplate and similar APIs are deprecated, not removed, in Spring Framework 7.1, so nothing breaks immediately. What changes is the risk profile going forward.

// still works after upgrading, but now emits a deprecation warning
RestTemplate restTemplate = new RestTemplate();
restTemplate.getForObject(url, Product.class);

Left unaddressed, the code accumulates compiler warnings that clutter build output and hide genuinely new warnings. It also misses out on features that only the newer client APIs understand - for instance, the framework's native version attribute for API versioning is understood by RestClient, WebClient, and @HttpExchange clients, but not by RestTemplate, so any code still calling downstream versioned endpoints through RestTemplate has to fall back to manually setting headers instead of using the declarative mechanism. Most importantly, the deprecated APIs are slated for outright removal in a future Spring 8 release with no announced date yet, so deferring the migration just shifts the work to a future, less-planned moment - likely under time pressure when the removal actually lands.