Debian builds Exim against GnuTLS because OpenSSL used to use a license with an advertising clause, making it incompatible with the GPLd Exim.
Since OpenSSL 3 is now available under a GPL-compatible license, I think it's long past time to switch. But judging by the sorry state of https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=446036 I don't think it's going to happen any time soon.
Color me surprised. The GNU ecosystem has had more than its fair share of CVEs over the years to the point that it's now a common trope:
https://soatok.blog/2020/07/08/gnu-a-heuristic-for-bad-crypt...