Now that SSLSocketFactory is deprecated on Android, what would be the best way to handle Client Certificate Authentication?

if you are using https, you have to use a valid certificate. During your dev stage you have to trust the certificate, how? sslSocketFactory(SSLSocketFactory sslSocketFactory) is deprecated and it's replaced by sslSocketFactory(SSLSocketFactory sslSocketFactory, X509TrustManager trustManager), you have to update your gradle file the piece of code below will help you to get a trusted OkHttpClient that trusts any ssl certificate.

TrustManagerFactory trustManagerFactory = TrustManagerFactory.getInstance(TrustManagerFactory.getDefaultAlgorithm());
trustManagerFactory.init((KeyStore) null);
TrustManager[] trustManagers = trustManagerFactory.getTrustManagers();
if (trustManagers.length != 1 || !(trustManagers[0] instanceof X509TrustManager)) {
    throw new IllegalStateException("Unexpected default trust managers:" + Arrays.toString(trustManagers));
}
X509TrustManager trustManager = (X509TrustManager) trustManagers[0];
SSLContext sslContext = SSLContext.getInstance("SSL");
sslContext.init(null, new TrustManager[] { trustManager }, null);
SSLSocketFactory sslSocketFactory = sslContext.getSocketFactory();
OkHttpClient client = new OkHttpClient.Builder().sslSocketFactory(sslSocketFactory, trustManager);

Apparently, there are two SSLSocketFactory classes. HttpClient has its own one, and that is deprecated along with the rest of HttpClient. However, everybody else will be using the more conventional javax.net.ssl edition of SSLSocketFactory, which is not deprecated (thank $DEITY).

Look this i find some solution and in my side is work well. Check how i've integrated..

OkHttpClient.Builder client = new OkHttpClient.Builder();

add here all properties for client instance

. . .

and add those line of code for sslSocketFactory:

 try {
        // Create a trust manager that does not validate certificate chains
        final TrustManager[] trustAllCerts = new TrustManager[]{
                new X509TrustManager() {
                    @Override
                    public void checkClientTrusted(java.security.cert.X509Certificate[] chain, String authType) {
                    }

                    @Override
                    public void checkServerTrusted(java.security.cert.X509Certificate[] chain, String authType) {
                    }

                    @Override
                    public java.security.cert.X509Certificate[] getAcceptedIssuers() {
                        return new java.security.cert.X509Certificate[]{};
                    }
                }
        };

        // Install the all-trusting trust manager
        final SSLContext sslContext = SSLContext.getInstance("SSL");
        sslContext.init(null, trustAllCerts, new java.security.SecureRandom());

        // Create an ssl socket factory with our all-trusting manager
        final SSLSocketFactory sslSocketFactory = sslContext.getSocketFactory();

        client.sslSocketFactory(sslSocketFactory, (X509TrustManager) trustAllCerts[0]);
        client.hostnameVerifier((hostname, session) -> true);
    } catch (Exception e) {
        throw new RuntimeException(e);
  }