Current messenger code uses STREAM_CRYPTO_METHOD_SSLv23_CLIENT to establish ssl connection to log in to SMTP servers, which only provides TLS v.1.1 and TLS v1..2.
TLS v.1.3 support was added to PHP streams since PHP 7.4.0RC1 (~ 6 years ago).
phpBB uses corresponding STREAM_CRYPTO_METHOD_TLS_CLIENT constant for PHP <5.6.7 which is not relevant for phpBB 3.3.