That's not really what they meant by "acceptable" - just that either of those metals are traditional.
What's "acceptable" depends on the people in the relationship, and what will make each of them happy.
I didn't have a ring when I proposed to my partner as it was a fairly spontaneous decision when we were in a foreign country, and I knew the location of the proposal would matter much more to her than a ring.
But then after we got back we worked together with a jeweller to design a ring, which made sense as she has better design judgment than me, and we weren't doing anything very lavish/expensive, just something tasteful that she would love (for the record it didn't have diamonds, but rather a morganite stone with white gold).
The conversation is framed from the point of view of tradition in some parts of the world. Of course the cops won't arrest you if you decide not to get an engagement ring. It's like going to work in flip flops, it's not illegal, just frowned upon in many circles.
These faux-intellectual "I'm a robot, beep boop I don't understand context and I parse conversations like a compiler would, semantic error on line 5" conversations are probably the least productive on this website, and they're unfortunately very common. It's not smart, it's just obtuse.
As in not having a ring is 'unacceptable' somehow then?