Qt 6.12, which supports deployment targets up to macOS 26, requires the macOS 15 SDK or higher.
See https://doc.qt.io/qt-6.12/macos.html.
This is a prerequisite for the upcoming update of Qt in depends from 6.8 LTS to 6.12 LTS.
My partial Guix build:
$ uname -m && find guix-build-$(git rev-parse --short=12 HEAD)/output/ -type f -print0 | env LC_ALL=C sort -z | xargs -r0 sha256sum
x86_64
727bcc7343afc599cadad3891a6aaa73883594a5feea6695b81056eaf1881914 guix-build-eaa8d1f5a5fc/output/arm64-apple-darwin/SHA256SUMS.part
cea76395e4761c9cc6c1e7fc0eae52abd3bb024caeed62a3461a9cf0de2f4b08 guix-build-eaa8d1f5a5fc/output/arm64-apple-darwin/bitcoin-eaa8d1f5a5fc-arm64-apple-darwin-codesigning.tar.gz
8886c05cca9f518441284be4cb415d275b7d118e8150e441d053e3fc61d27807 guix-build-eaa8d1f5a5fc/output/arm64-apple-darwin/bitcoin-eaa8d1f5a5fc-arm64-apple-darwin-unsigned.tar.gz
7c6c4423e6272a821fd6adefaac6dd4a1e09f7f85ecf9efa6bd7e97c0ddc79bc guix-build-eaa8d1f5a5fc/output/arm64-apple-darwin/bitcoin-eaa8d1f5a5fc-arm64-apple-darwin-unsigned.zip
a74db20b1a8acd1a570a733d02ea7e3aa28ac8902462e9be9fac10e0f84e4ca1 guix-build-eaa8d1f5a5fc/output/dist-archive/bitcoin-eaa8d1f5a5fc.tar.gz
f708d94dc0343a5057d3b8791242acd67dbb1f481b2da7fc778aa981e9fc2e42 guix-build-eaa8d1f5a5fc/output/x86_64-apple-darwin/SHA256SUMS.part
98ddcc471b967208e491c06f697e94d09005631da463e085ee88e281c035180e guix-build-eaa8d1f5a5fc/output/x86_64-apple-darwin/bitcoin-eaa8d1f5a5fc-x86_64-apple-darwin-codesigning.tar.gz
7aeb9b6f84679f11d22ffe1faf4b0aa386c09f641a6d742f70ef727466b24d0f guix-build-eaa8d1f5a5fc/output/x86_64-apple-darwin/bitcoin-eaa8d1f5a5fc-x86_64-apple-darwin-unsigned.tar.gz
de23e46dbe3c0886fd689b16ab345120907445212ffb6fcb41ccca8cfc8d628e guix-build-eaa8d1f5a5fc/output/x86_64-apple-darwin/bitcoin-eaa8d1f5a5fc-x86_64-apple-darwin-unsigned.zip