TLS 1.3 was introduced back in August 2018.   However, Exchange 2019 and Exchange Online do not currently support it.   Windows 2022 has introduced support for TLS 1.3, but Exchange 2019 does not support it, so please do not try this as likely will cause issues for Exchange 2019.  You need to remain using TLS 1.2 within Exchange 2019 and Exchange Online until Microsoft provide a formal update that it is fully supported. 

 The latest Exchange 2019 CU update is CU14, and this does not provide the TLS 1.3 support.   It is widely expected that Exchange 2019 CU15 will provide TLS 1.3 support. 

https://datatracker.ietf.org/doc/html/rfc8446

 Some highlights for TLS 1.3 when it is supported by Exchange 2019 and Exchange Online. 

•  TLS 1.3 is faster than TLS 1.2 – mainly because the SMTP handshake only requires one round-trip, not two.  Latency is reduced as a result. 
•  TLS 1.3 drops support for a number of less secure cryptographic algorithms, making TLS 1.3 less vulnerable to cyber attack. 

 Please contact us if you required any consulting help with your Exchange Hybrid or Exchange Online environment.