Soul

In the Qur'an, when used alone, the word "nafs" is not really used as a synonym for soul (in the sense of the soul as a non-corporeal entity distinct from the body). Rather, it is usually used to refer to the complete self; for instance, in phrases like "bring yourselves" or "they deceive themselves".

(Of course, this distinction can be blurry based on how one perceives the nature of the soul and the relationship between the soul and the body.)

This is different from the use of the word "nafs" outside the Qur'an, in that "nafs" in other texts or discussions is often used to mean specifically the immaterial part of the human.

Therefore, when the Qur'an says "kullu nafsin dha'iqat al-mawt", it means that all people will taste death in a holistic sense, not specifically that all souls will.

However, since the soul and body experience things simultaneously, the soul will also taste the experience of death, just as it experiences other things occurring via the body.

If one understands death to mean "transfer from realm to realm", rather than "annihilation", then definitely the soul will experience transfer from realm to realm upon death.

Some say that we will have a different type of body as a vehicle for the soul in the barzakh and in the Hereafter. Therefore, if death is also understood to mean "transfer from one type of body to another", the soul will also taste death in that way.

So it is not incorrect to say that all souls will taste death; the usage of the word in the Qur'an is simply more expansive than that.

Note that compound phrases in the Qur'an such as nafs al-'ammarah (the bestial soul), nafs al-lawwamah (the blaming soul), and nafs al-mutma'innah (the content soul which God calls to Paradise) refer more to immaterial aspects of the human being.

Here are a few narrations on this topic:

Imam al-Sadiq (A) said, “The things which enter a person’s grave are ritual prayer, fasting, hajj, charity, good deeds, supplication, and its reward is written for those who do them and for the deceased.” (Man la yahduruhu al-faqih)

Imam al-Sadiq (A) said, “Whoever of the Muslims does a good deed for a deceased person, God will double his reward and benefit through it the deceased” (Bihar al-Anwar citing Uddat al-Da’i)

It is related that a man came to the Prophet (S) and asked, “My mother has died. Will it benefit her if I give charity on her behalf?” He said, “Yes.” (Musnad Ahmad)

Apart from Wahhabis who object to this because they see everything as bid'ah, it is hard to see the objection to this since if one is doing something good in the hopes that it would benefit the deceased, at least one is doing something good and beneficial. 

Maybe some people might object because some things are done for esaal-e-sawab which are not overly beneficial in the present time, for instance, printing Qur'ans and distributing them in an area where Qur'ans are easily accessible. This would have been very beneficial in earlier centuries but today because printing is cheap, it often is not the most beneficial thing to do even if it is meaningful, or sometimes things could even be wasteful. So it is good to think about what is most beneficial, and I am sure the deceased appreciate being remembered in a good way!