This has been growing "wild" in behind my shed for years. Doesn't seem to get taller (it's 2ft tall, approx), doesn't flower or change colour, or grow flowers. I thought the leaves looked like Spirea, but not sure. I don't water it or take care of it, but it's just there.

You should include some pictures with your question here; that always helps the crowd.

That said: you posted on another forum with same question plus a picture. I'll copy my answer here.
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I think you have a mimic there.

Ural False-spirea - Sorbaria sorbifolia is very cold hardy, but not known for being excessively showy. It makes its best use as a spreading/colonizing ground cover with fine/feathery foliage habit.

Ural False Spiraea (Sorbaria sorbifolia)

Sorry, thought I did. Here it is.

It doesn't change colour, and doesn't have flowers. It looks rather sparse and gets no more foliage that you see here.

ViburnumValley said:
I think you have a mimic there.

Ural False-spirea - Sorbaria sorbifolia

That sure looks like it.

Here's a better picture of it