Each and every line of code you write is a liability. Even more so when you wrote it for someone else. You must always be able to rebuild it from source, at least as long as your client expect the software to work. If you feel it’s not worth it, you probably low-balled the contract. If you don’t want to maintain code, have the client pay a yearly maintenance fee, give the code and the responsibility to maintain it to your client at the end of development, or add a time limit to it’s support.

There’s no “maintenance mode” software: either it’s in use and must be kept updated with regard to it’s execution environment, or it’s not used anymore and can be erased and forgotten. Doing differently opens too much security issues, which shouldn’t be acceptable for us all as a trade.