Is a postdoc-interview presentation a norm?

At least in my field and country, giving a talk about your previous work is pretty common for the hiring process of any academic position. But that does not mean you should prepare a talk. Nobody can seriously expect you to know this and automatically bring a talk to a job interview just because it’s the unwritten norm (and you probably do not want to work with the people who do). Moreover, it is impossible for you to deduce the desired length, audience, scope, and other aspects of the talk.

Instead, you have to estimate how likely it is that this aspect was just forgotten in the invitation (e.g., going by the other details it contains). If you think that there is any non-negligible chance that this is the case, just ask them.

In my field (theoretical computer science) either way is normal: some interviews include a presentation, some don't.

If they haven't told you that you'll be making a presentation, the natural assumption is that you won't. But there's no harm at all in asking them in a short email. Obviously, you'll spend some of the interview talking about the work you've done so far, but discussing your work is completely different from giving a presentation and it would be a waste of time to prepare a presentation that isn't going to get used.

There are two ways you can handle this, and both of them are good options. But quite frankly, if I were in your place, I'd take the second option.

First, you can ask them now whether they will want you to be ready for a presentation. This will start presenting you as a good communicator, and that is a big plus. This also takes the guesswork out of the process. Most likely they will give you a definite answer (although I would not), and the question will be settled.

Second option, and in my mind the most desirable, is to go in loaded for bear. Be prepared to give a full presentation, a short presentation, and a summary of your previous work which is most likely to be of interest to them, and at least a thorough summary of any other work you have done. They will inevitably ask about your past work. At which point you can say, "well, I think project x would be most interesting to you. I can give you a quick summary, a short presentation, or a full presentation, what would you prefer...". They will be impressed with your preparation, and might be non-committal with their answer. Unless they tell you exactly, go with the short presentation and adjust up or down in detail as you see their interest peak or wane.

One of the big things in any interview process is to have some WOW! factor, something that sets you apart from other applicants and leaves your interviewers unable to forget you. The fact that they have left the need for a presentation unspecified gives you a great opportunity to have a big time WOW! Why erase that by asking ahead of time?