This maintenance release of libpqxx works around a <thread> bug in MinGW; fixes some Visual C++ warnings; and makes the code a little easier to build out of the box by including config/compile in revision control.
But also, it deprecates more "legacy" representations of binary data, such as unsigned char *. We keep moving towards std::byte. If your compiler supports it, upcoming releases will move towards generic support for spans and ranges of binary data. If not, you'll need to represent binary data as std::basic_string<std::byte> and std::basic_string_view<std::byte>.
Eventually we'll raise the baseline C++ version to C++20, and at that point, it will be all ranges and spans.