Because from what Iv'e seen on the marketplace (don't quote me on that though), most of the plugins are, in fact, updated to 4.26, and those that aren't, are not (at least in my view) worth your money (As that their developers don't bother to maintain their products).
If you are using pirated assets for a commercial game (As I understand from you, deriving from your mention of "serious production", and given the fact that you are posting in this forum),
I would highly recommend against it (Not only does it leave you open for lawsuits, it is also highly unethical).
At any case, I would recommend you upgrade to 4.26 before starting a fully-fledged project, and worry about backwards-compatibility later
