Ye, it’s really something you can only get from talking to (network) engineers on the project.
You can work with it always in the back of your mind, trying to make decisions to ensure it remains a possibility (assuming you’re not tied to external or pre-existing code that would preclude it). But it’s going to be something that will have been constantly considered during development if it is something that could be implemented at the end (as a reasonable patch). The entire project would need to be making decisions where it was considered (even if built on top of an engine that definitely already supported it when the project started).
Take something simple like a library of maths functions (included with every SDK with each platform), optimised with the fast way of doing various calculations (chosen based on both the hardware and the types of choices they’re expecting most developers will want to make on that platform). If you’ve got a game to ship then maybe you don’t need the exact results of the IEEE standard and some calculation could be “mostly right” (note that limited precision means floating point maths isn’t the same as actual mathematics anyway so it’s not actually shocking to pick doing things faster with less accuracy, that’s already inherent in the limited precision of calculations used throughout games). Now do all platforms have the same compiler & optimisations meaning the same decisions being made for those faster calculations and do they all map to the same hardware functions that will mean that the calculations are identical on each platform?
If you’re playing a multiplayer game then one of the things that is extremely useful is that if I do a calculation on my client and another client does the same calculation, we always get the same value out of it. That way we don’t have to synchronise our results because if we started with the same numbers we know we’re still synchronised after doing the calculation. If not however… slowly we could start to have different ideas about what the shared world we’re in actually looks like, about if we’re both agreeing to the same rules of the game we’re playing. Even when the clients are identical and the game is lock-step (everyone does all the same calculations all the time) then it’s hard work. Without guarantees around clients acting identically, the engineering job gets even harder.
That’s just a tiny example of one of the many things that can go wrong and would make implementing cross platform play extremely difficult after having already built a game. It’s not that necessarily any one thing would be impossible to patch, it’s that every inch of the game may not have been constructed always considering the possibility and so there are a million different spots that you’d want to check to ensure everything was right.