I have not needed to change this project in quite a while. I now need to a a single line, however, it does not compile, multiple definition of __vector_25, multiple definition of __vector_26,  multiple definition of __vector_23,  multiple definition of digital_pin_to_bit_mask_PGM, multiple definition of digital_pin_to_port_PGM, ...
I removed the new line and I get the same errors.
I Enabled vMicro > Compiler > Show Build Properties and re-compiled with new line restored.
I then discovered that I did not have the latest installed, installed 2.2024.1211.6, recompiled only to find the same errors.
Please find attached a copy of the output.

In addition, I've update Arduino IDE to 2.3.4.
Release version -> same errors.
Meanwhile, I used the Arduino IDE by using VS2022 to create a new project, Test_20250106, replaced the src folder with the KeyboardMega_2024_10_01 src folder and replaced the contents of Test_20250106.ino with the contents of  KeyboardMega_2024_10_01.ino. At first it could not find the #include. I update the include to the full path and it compiled and uploaded!
vMicro 

2 files

Thanks for the project.

When you selected the Release Build Configuration, was the Serial Debugger set to "Off"?

I only ask as the errors definitely seem to be a result of the Serial Debugger being enabled.