This seems to be a patch for the binary. Which assembler was used to compile it and how do I apply it?
I had expected a source-code patch, since both cc65 and PLATOterm (AFAIK) are available in source.
FYI, in case you don't know: If you want to modify some part of the cc65 runtime library (I think the serial driver is changed by this patch, at least), you can copy the relevant source file from the cc65 library, modify it to your needs, and use this version. So copy e.g. libsrc/atari/ser/atrrdev.s to your directory, modify it to your needs, and compile like this
cl65 -t atari .... atrrdev.s ...
As long as you don't change the exported symbol names, the linker will pick up your modified version instead of the version contained in the lib.
Remember, the cc65 lib must work for general programs. You know what your program does. And if the official cc65 lib version is suboptimal (because of checks for things which cannot happen in your program), it's perfectly valid to use a local tweaked version of the lib file(s) affected.