diff --git a/.semaphore/semaphore-integration.yml b/.semaphore/semaphore-integration.yml
index 5402904c4..a968b0c3c 100644
--- a/.semaphore/semaphore-integration.yml
+++ b/.semaphore/semaphore-integration.yml
@@ -89,7 +89,9 @@ blocks:
           type: s1-prod-ubuntu24-04-amd64-1
       env_vars:
         - name: CFLAGS
-          value: -std=gnu90 # Test minimum C standard, default in CentOS 7
+          value: -std=c99 # Test minimum C standard
+        - name: CXXFLAGS
+          value: -std=c++98 # Test minimum C++ standard
       prologue:
         commands:
           - '[[ -z $DOCKERHUB_APIKEY ]] || docker login --username $DOCKERHUB_USER --password $DOCKERHUB_APIKEY'
diff --git a/.semaphore/semaphore.yml b/.semaphore/semaphore.yml
index c43061ed3..3924178d8 100644
--- a/.semaphore/semaphore.yml
+++ b/.semaphore/semaphore.yml
@@ -127,7 +127,9 @@ blocks:
           type: s1-prod-ubuntu24-04-amd64-1
       env_vars:
         - name: CFLAGS
-          value: -std=gnu90 # Test minimum C standard, default in CentOS 7
+          value: -std=c99 # Test minimum C standard
+        - name: CXXFLAGS
+          value: -std=c++98 # Test minimum C++ standard
       prologue:
         commands:
           - '[[ -z $DOCKERHUB_APIKEY ]] || docker login --username $DOCKERHUB_USER --password $DOCKERHUB_APIKEY'
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index e6afdc1ea..5006573e1 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -250,21 +250,12 @@ E.g.:
 *Note: The code format style is enforced by our clang-format and pep8 rules,
 so that is not covered here.*
 
-## Minimum C standard: "gnu90"
+## Minimum C standard: "c99"
 
-This is the GCC default before 5.1.0, present in CentOS 7, [still supported](https://docs.confluent.io/platform/current/installation/versions-interoperability.html#operating-systems)
-up to its EOL in 2024.
+C99 is fully implemented (except VLA) since Visual Studio 2013 and it's the
+default starting from GCC 5.1.0 (available since CentOS 8).
 
-To test it, configure with GCC and `CFLAGS="-std=gnu90"`.
-
-It has the following notable limitations:
-
- * No in-line variable declarations.
-
-**Note**: the "No variable declarations after
-  statements" (-Wdeclaration-after-statement) requirement has been dropped.
-  Visual Studio 2012, the last version not implementing C99, has reached EOL,
-  and there were violations already.
+C++ standard is C++98.
 
 ## Function and globals naming