The NRPE code has config options for TLS 1.3 that is not documented in nrpe.cfg.in
I think the TLS 1.3 support was released in 4.0
# SSL VERSION
# This can be any of: SSLv2 (only use SSLv2), SSLv2+ (use any version),
# SSLv3 (only use SSLv3), SSLv3+ (use SSLv3 or above), TLSv1 (only use
# TLSv1), TLSv1+ (use TLSv1 or above), TLSv1.1 (only use TLSv1.1),
# TLSv1.1+ (use TLSv1.1 or above), TLSv1.2 (only use TLSv1.2),
# TLSv1.2+ (use TLSv1.2 or above)
# If an "or above" version is used, the best will be negotiated. So if both
# ends are able to do TLSv1.2 and use specify SSLv2, you will get TLSv1.2.
# If you are using openssl 1.1.0 or above, the SSLv2 options are not available.
The NRPE code has config options for TLS 1.3 that is not documented in nrpe.cfg.in
I think the TLS 1.3 support was released in 4.0