I'm having a similar lock-up using Objective-C++ ; I'm not sure the C++ portion is relevant, but I don't think that Swift is required for the problem. The test I'm seeing hang has been in my codebase since June 2009 with no recent changes. I do link with some Swift code, but it isn't used in these tests. I have a number of tests that run as part of my build process and I've not had problems before 14.3. However, with this version it's about a 50% chance that one of my test binaries will lock up when run from the GUI. Running through fastlane or directly through xcode-build. I do not see these problems on M1 or M2 Macs in our build farm, but I do see it on my x86_64-based MacPro. I've filed Feedback #FB12154691 on this issue.

It appears this may be more isolated than it first appeared. Definitely looks like libpq is gone from both the 10.16/11 SDK and /usr/lib on machines that are running new installs of macOS 11. However, libcurl, libxml2 and libxslt (and others) still have tbd files and appear to be available under 10.16/11 without a problem. It does appear that if you link against libpq.tbd, not only do you need to handle incorporating the libraries directly, but you need to issue an update to make it compatible for running on macOS 11. However, with this change it looks like our app is starting up on macOS 11. I'm still looking for a better answer if anyone has one.