If it's been written with a view to be mod friendly, it can still have no mods if it's not a popular game. If a game is tightly sewn up, encrypted and compiled, then it's going to be harder to deconstruct what's going on in order to mod the thing.

However, whether they are is down to two things - ease of access and popularity. So yes, in theory everything that is code can be modded. I have several lever arch files full of my own codes on various platforms dating back over 35 years. I 'mod' many old games on various consoles just by plugging away changing memory addresses. Whether it can be easily changed is the real question. All games are code and as such can be changed.