After the recent full removal of Autotools (PR #30664), these macros are not needed anymore in the .cpp files according to the TODO in qt’s CMakeLists.txt. Tested building on OpenBSD 7.5, where the XCB plugin was still imported according to the debug log:
02024-09-02T21:13:27Z Bitcoin Core version v28.99.0-7346b0109208 (release build)
12024-09-02T21:13:27Z Qt 5.15.12 (dynamic), plugin=xcb
22024-09-02T21:13:27Z No static plugins.
32024-09-02T21:13:27Z Style: fusion / QFusionStyle
42024-09-02T21:13:27Z System: OpenBSD 7.5, x86_64-little_endian-lp64