Everything worth knowing, nothing that isn't.
What was removed
TLS 1.3 drops RSA key exchange, static DH, CBC-mode ciphers, MD5 and SHA-1 in the handshake, compression (the CRIME class of attacks), and renegotiation. What remains: ECDHE for forward secrecy, AES-GCM / ChaCha20-Poly1305 for AEAD, and SHA-256/384 for transcript hashing. The cipher suite list went from dozens to five. This is a feature.
What changed in the handshake
TLS 1.3 does a single round-trip for the full handshake (down from two), and supports 0-RTT resumption for repeat connections. On an SMPP transceiver bind that reconnects every few hours, the saved RTT compounds. More importantly, the simpler handshake removed the downgrade-attack surface that plagued TLS 1.2 (POODLE, FREAK, Logjam).
What SMPP customers should check
Three things. Your client library supports TLS 1.3 (most do as of 2022; check OpenSSL >= 1.1.1, JSSE on JDK 11+, Go 1.12+). The SMSC accepts TLS 1.3. The cipher list on your side includes at least one of `TLS_AES_128_GCM_SHA256` or `TLS_CHACHA20_POLY1305_SHA256`.
Where TLS 1.2 still lingers
Some legacy SMSC stacks (Acision running on RHEL 5, certain Huawei nodes) cap at TLS 1.2 because the underlying OpenSSL is end-of-life. We negotiate down where required, but for any new bind we require TLS 1.3. For PDPL audit purposes, all production SMPP at BroadNet is on TLS 1.3 with cipher pinning.