TL;DR: swift-nio-ssl is unaffected.
OpenSSL has published a security advisory that discloses a High severity security issue that partially affects BoringSSL, CVE-2023-0286. This issue only affects BoringSSL users that enable CRL checking, by setting X509_V_FLAG_CRL_CHECK
.
swift-nio-ssl does not and has never set this flag, so it is unaffected. No action is required.