As described in the RESTEasy document - 32.1. Exception Mappers, the default ExceptionMapper is enabled since RESTEasy 6.1.0+ by RESTEASY-3097, and it can be turned off by setting the "dev.resteasy.exception.mapper" to false since RESTEasy 6.2.0+ by RESTEASY-3225.

Setting the "dev.resteasy.exception.mapper" to false through the system property (= adding -Ddev.resteasy.exception.mapper=false as JVM option) works fine without RESTEasy MicroProfile, but setting the "dev.resteasy.exception.mapper" to false through <context-param> in web.xml (like the following) does not work without RESTEasy MicroProfile.

<web-app>
    <context-param>
        <param-name>dev.resteasy.exception.mapper</param-name>
        <param-value>false</param-value>
    </context-param>
</web-app>

As far as I checked the RESTEasy document - 3.4. Configuration, it says:

If MicroProfile Config is not available, then an attempt is made to retrieve the parameter from the following sources in this order:

1. system variables, followed by
2. environment variables, followed by
3. web.xml parameters, as described in Section 3.4.5, "RESTEasy's classic configuration mechanism"

Therefore, I think that <context-param> in web.xml should work even if MicroProfile Config is not available.