MEDIUM: config: warn when expose-experimental-directives is used for no reason

If users start to enable expose-experimental-directives for the purpose
of testing one specific feature, there are chances that the option remains
forever and hides the experimental status of other options.

Let's emit a warning if the option appears and is not used. This will
remind users that they can now drop it, and help keep configs safe for
future upgrades.

doc/configuration.txt

@@ -2464,7 +2464,10 @@ expose-experimental-directives
   breaking them but with no guarantee. For these reasons, these features are
   not expected to be supported beyond the release of the next LTS release.
   Users who want to try experimental features are expected to upgrade quickly
-  to benefit from the improvements made to that feature.
+  to benefit from the improvements made to that feature. In order to know if
+  this directive is still needed, it's easy: if it is enabled without being
+  used by any such feature, a warning will be emitted suggesting to turn it
+  off. So without any warning, it means it's still needed.
 
 external-check [preserve-env]
   Allows the use of an external agent to perform health checks. This is

src/cfgparse.c

@@ -4609,6 +4609,14 @@ init_proxies_list_stage2:
 			cfgerr += postparser->func();
 	}
 
+	if (experimental_directives_allowed &&
+	    !(get_tainted() & TAINTED_CONFIG_EXP_KW_DECLARED)) {
+		ha_warning("Option 'expose-experimental-directives' is set in the global section but is "
+		           "no longer used. It is strongly recommended to remove it in order to avoid "
+		           "using an experimental directive by accident in the future.\n");
+		err_code |= ERR_WARN;
+	}
+
 	if (cfgerr > 0)
 		err_code |= ERR_ALERT | ERR_FATAL;
  out: