There are several upstream dependencies that you need to consider in your code. These come in the form of your operating system, the platform your application or service runs on, the external applications and services you use, and the contracts you depend on for data.
Operating systems and software platforms can affect the performance, security, and ability of your service. Writing code that lasts means abstracting your code enough from the OS and underlying platform such that you can upgrade the OS or platform without changes to your application or service. This allows you to protect your self and your customers from security vulnerabilities as well as allows you to take advantage of performance enhancements that come from kernel or library updates.
You should also consider the dependencies you have an external applications or services. Do you depend on them in a version specific way? How will your application need to change if they change the schema of the contract you have or deprecate an API you use. Building code that will last doesn't mean prematurely optimizing your code to try to defend against these changes, but instead means understanding how to encapsulate them such that the effect these changes have on your code are minimal.
Downstream dependencies are people, applications, and services that depend on your software. From a downstream dependency perspective writing code that will last means understanding when you're making a choice that you can't go back and change after the fact. In order to write code that lasts we have to minimize the number of these choices that we make. These choices often come in the form of the APIs we expose, the contracts we create with other services, the way we represent objects, and etc. Once these choices are made, sometimes they can't be taken back without having some sort of negative side effect.