ECParameters ::= CHOICE {
namedCurve OBJECT IDENTIFIER
implicitCurve NULL
specifiedCurve SpecifiedECDomain
}
PKIX, which is what TLS and most other standards use for ASN.1 message grammars, already mandates that "implicitCurve and specifiedCurve MUST NOT be used". See https://tools.ietf.org/html/rfc5480#section-2.1.1 (There are many other related RFCs. It gets very confusing, especially once you take into account obsolete and draft RFCs.)Newer curves, like EdDSA curves, are always each specified with unique OIDs and have a simpler public key grammar. See https://tools.ietf.org/html/rfc8410 Older public curves share an OID and a more generic ("flexible") syntax, thus the ECParameters field. (RSA public keys also have a parameters field, but it's unused. However, annoyingly, some implementations omit the field altogether, others set a NULL value.)