The SunJSSE probes SHA1withECDSA Signature availably when determining if elliptic curve cryptography is available. See the EcAvailability class in JsseJce.java. https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/sun/security/ssl/JsseJce.java#L168

SHA1 is now deprecated, and some JCE providers no longer ship SHA1withECDSA.

If no provider that provides a SHA1withECDSA Signature algorithm is registered then the SunJSSE infers that elliptic curve cryptography is not available. All cipher suites that involve elliptic curve cryptography are dropped.