Should serialized models be compatible between versions

Should I expect a model serialized from xgboost version 1.0.0 to be readable by versions 0.82 and 0.90?

I have a case where it is not - I’m trying to figure out if I should file an issue about this, or whether this isn’t something that I should expect to work.

No, you should not expect it to work. Let’s make an analogy: While a Blu-Ray drive can read a CD, a CD drive cannot read a Blu-Ray disc. In the same way, you can migrate models from 0.90 to 1.0.0 but not the other way around.