WebClient vs RestTemplate

RestTemplate is not really deprecated. But it will not be evolved in the future. So sticking to RestTemplate is perfectly valid if it does what you need.

Another way to put that is that if you need specific usage patterns like streaming, scatter/gatter, or custom timeouts, this won't be covered by RestTemplate and you need to use WebClient instead.

Now using WebClient in a blocking application is fine too. Using block() shouldn't hurt there and Spring MVC controller does partially support reactive return types.

No, RestTemplate will continue to exist (at least for now). You don't have to replace it with WebClient.
One of the main differences is RestTemplate is synchronous and blocking i.e. when you do a rest call you need to wait till the response comes back to proceed further.

But WebClient is complete opposite of this. The caller need not wait till response comes back. Instead he will be notified when there is a response.

If you need such a functionality, then yes you need to replace your Resttemplate with WebClient.
You can in fact achieve Rest template like synchronous processing in webclient using .block(). But the other way is not possible.

EDIT:

RestTemplate will be deprecated in a future version(> 5.0) and will not have major new features added going forward

WebClient is Non-Blocking Client, RestTemplate is Blocking Client.

For a long time, spring serves as a web customer. Under the hood, RestTemplate uses the Java API API, which is based on the subject model.This means that the matter will be blocked until the client receives a response. The problem with the blockage code is due to the existence of any string of memory and cpu cycles. Let us consider a lot of applications that are waiting for low services that are needed to produce the result.Sooner or later, requests for the results are collected. As a result, the program creates many issues, which result in depletion of a pool of thread or occupying all of the available memory. We can also experience performance performance due to the cpu switching.

Spring WebClient vs. RestTemplate

According to the Java Doc the RestTemplate will be in maintenance mode. Spring team advise to use the WebClient if possible:

NOTE: As of 5.0, the non-blocking, reactive org.springframework.web.reactive.client.WebClient offers a modern alternative to the RestTemplate with efficient support for both sync and async, as well as streaming scenarios. The RestTemplate will be deprecated in a future version and will not have major new features added going forward.