In technical writing (and indeed in most expository writing) the best rule I've found to be useful is tense consistency.
So if you say
foo was once designed and implemented in a way called bar that required X to do Y
you should continue along the lines of
In this scheme, bar caused farf to jiff and klurg was responsible for regulating the status of kling..."
i.e., continue with the past tense.
Then, when you move to describing the new way of doing things, you can choose present or even future. They have different connotations. Here is the future option:
In the new system, foo will be implemented by first jiffing and then kluring the inputs, after considering the status of bar. Bar will take the place of shlimpf when vlerg finishes.
Choosing the future case requires that the implementation work has not yet been done, and maybe that the whole setup is open to revision. But you can also use the present tense in this case, especially if you are trying to resist revisions. And you would also use the present in the case where the implementation has already been done:
In the new system, foo is designed and implemented by first jiffing and then klurging the inputs, after considering the status of bar. Bar takes the place of shlimpf when vlerg finishes.
(Notice that the present tense of passive verb forms end in -"ed"; it's the auxiliary (is vs. was) that determines the tense of passive forms.)
Could you use the past tense as well? Consider
In the new system, foo was designed and implemented by first jiffing and then klurging the inputs, after considering the status of bar. Bar took the place of shlimpf when vlerg finished.
There are two problems here. The bigger one is that the choice of tense in the second sentence is incorrect. The second sentence describes a continuing tendency, rather than a result (something done and over with), and so by the general rules of English grammar the past tense is prohibited. But then the first sentence sounds too disjoint with respect to the second if you leave it in the past tense, so converting it too to the present tense improves the whole thing.
The second problem with retaining the past tense is that even if you dropped the Bar sentence, the effectiveness of any contrast you are making between the past way of doing things and the present way of doing things is undermined by sharing the same tense between the two discussions. It is easier to confuse the two, especially when jumping between different parts of a document.
Finally, if you don't discuss the earlier system at all, the present tense is generally preferred anyway, on the basis that it makes descriptions more vivid and engaging than the past tense. Though it can feel manipulative when used to construct narratives of one-time events, it never has this feeling in the kind of technical writing you are describing.
How has the role of the verbs changed?
The verb form "written" is a participle that happens to be named past participle. It has no tense: no present tense nor past tense. This also holds for the other participle verb form that happens to be commonly known as the present participle (also known as gerund-participle).
(Aside: It so happens that the past participle could be connected to a secondary past-tense when it is used in a perfect construction. But that is due to the perfect construction.)
If a participle is used in a non-finite construction, then a "tense" could be borrowed from a superordinate clause. The role of the verbs in both of your examples haven't really changed much at all. In both examples, the "tense" of the verb is is being borrowed by the clause that is headed by the verb written when the written clause is being interpreted. (Syntactically there are differences between the two sentences, but I don't think you are asking about those differences.)
Here's a way to parse the two sentences, with a clause in brackets "[ ]", and with the head verb of each clause bolded:
If you want a traditional grammar type of explanation, well, er, someone else can give you one of those. :)
Best Answer
In this case is more a matter of aspect than tense.
English has both tense and aspect, but encodes them ambiguously, using the same words, particles and morphemes for both.
If you use the past tense for the subclause like this: "I asked mum how come my sister didn't feed him." Then you are also making the aspect a single action at a single point in time, instead of the habitual aspect that is needed here.
The most important thing in this context is that it's still possible, when the whole sentence is spoken, for the little sister to feed them. The concept that (s)he asked his/her mum about is still current. So in the subclause the habitual aspect is given through the simple present form.
If you use the perfect it's possible to make it past tense and continuous aspect: "I asked mum how come my little sister hasn't been feeding them" but then you lose some of the emphasis that it's still possible for her to be doing it now.