The Garbage Collection page of the spec (https://dlang.org/spec/garbage.html) mentions that it is undefined behaviour to store pointers to GC memory in non pointer types. Is this a relic of the past? I've seen a lot of people here on the forum saying that the GC needs to be conservative because D is a systems programming language. Wouldn't it be legal to make a Garbage Collector that uses type info, since the language allows it?

I think it should be noted though that druntime states that memory is always conservatively scanned: https://dlang.org/phobos/core_memory.html

AFAIK, precise GC was tried not long ago, but was only a little slower or something like that.