BigFix is looking at adding support for TLS 1.3 as a priority, with no committed plan as of today.
If your environment is configured to block TLS 1.2 communication then yes, this will break BigFix.
While TLS 1.3 provides significant enhancements in terms of security and performance, we are not aware of an urgent reason to block TLS 1.2. Do you have reasons/plans that you want to share?
Alessandro Dinia, BigFix Product Manager