I was just wondering in the following example whether it is necessary to use which. [...]
e.g. In support of your contention that you acquired units in the ABC unit trust, you provided the following documents:
a) Swiss bank history report which shows on 1 January 2020, you withdrew an amount of $100K to acquire the units in the said trust.
Hello, Tony C—In that sentence, "which shows" marks the beginning of a relative clause in which "which," a relative pronoun, functions as subject. When a relative pronoun functions as the subject of a relative clause, it cannot be omitted. You may, however, if you like, delete "which" and replace it with "that."
There are a few problems with (a). First, since "report" is a count noun, you should use "a" at the beginning: "a Swiss bank history report." Second, you can delete "an amount of." Third, you should introduce the "that"-clause after "shows" with "that" or move "on 1 January 2020" so that it comes after "$100K."
a1) a Swiss bank history report which shows that, on 1 January 2020, you withdrew $100K to acquire the units in the said trust
a2) a Swiss bank history report which shows you withdrew $100K on 1 January 2020 to acquire the units of the said trust
a3) a Swiss bank history report showing you withdrew $100K on 1 January 2020 to acquire the units of the said trust
Option (a3), in which "which shows" is replaced by "showing," shows that you can use a reduced relative clause instead instead of the finite relative clause. Please note I've deleted the period at the end and not capitalized the first word. That is because (a), (a1), (a2), and (a3) are not sentences. They're noun phrases.