Learn English – “Have completed” or “had completed”

Essentially, the two sentences express the same thing: people who do not have their belongings anymore will turn into miserable people. For most people they will be interchangeable. There is a slight difference in the emphasis.

Using the present perfect emphasizes on the result. It talks about people who have had their things taken before now, at any point in the past; it doesn't matter when exactly, what matters is that they do not have them now. The result is important.

Using the past simple simply talks about an action at a moment in the past. In this case I would think we are talking about one particular occasion when someone took their belongings because I do not see anything in the sentence that implies repetition.

Whether to use "have had" or "had" depends on the context. If you are talking about a time when many things happened, one of which was that people had their belongings taken, then I would use the past simple because there is one particular past moment we have in mind. If there isn't one, and we want to say: "people who do not have their belongings now", then I would choose the present perfect.

About the second pair, you are right in your assumptions. Let me expand the phrases. The first one is: "people who normally/usually smoke". the second one is: "people who have ever smoked before/in the past (whether regularly or even once)".

The requested modifications have been completed.

is better, because you are referring to a continuing action (you finished writing the code, but it will get tested next).

Put into context:

The requested modifications have been completed. You may now begin your testing process.

In this example:

The requested modifications are completed.

are refers to the current state, but completed is past tense. I won't say people don't write it this way, but it would be better to write:

The requested modifications are complete.

As a simple statement of fact this is fine.