Some grammarians and style guides recommend hypenating multi-word adjectives where they precede a noun. The exception to this 'rule' is when the adjective is used after the noun (i.e. postpositively). So, no, zero-rated is not incorrect.

A postpositive or postnominal adjective is an attributive adjective that is placed after the noun or pronoun that it modifies. This contrasts with prepositive adjectives, which come before the noun or pronoun.

https://en.wikipedia.org/wiki/Postpositive_ad...

Hier noch mal Wikipedia dazu, als zusätzlicher Beleg.

Und zwei Anwendungsbeispiele zur Illustration:

The mysterious world of zero-rates of VAT - BBC News

28.10.2015 - This means that pistachios are usually regarded as zero-rated products

https://www.bbc.co.uk/news/magazine-34649495 

VAT Relief – EnableMedical. You will receive products zero rated so you will not be charged VAT.

https://enablemedical.co.uk/vat-relief/

Wobei man im Netz auch jede Menge Beispiele findet, bei denen das zero-rated mit (!) Bindestrich nach (!) dem Bezugsobjekt steht. Das scheint also keine strenge Regel oder jedenfalls eine, die in der Praxis oftmals nicht beachtet wird.

@ 4: Kein Bindestrich, würde ich sagen (non-native und auch kein Grammar-Freak).

Hyphens and Adverbs

Although it’s OK to use hyphens with adjectives, hyphens and adverbs don’t get along as well. You shouldn’t use hyphens with adverbs such as “happily” and “individually.” For example, you don’t put hyphens in phrases such as “happily married man” and “individually wrapped cheese.”

https://www.quickanddirtytips.com/education/g...

In your specific examples, I agree with mad. In general, though, it depends ;-)

The Chicago Manual of Style, for example, recommends:

An adverb ending in -ly followed by a participle or adjective is always open (i.e. unhyphenated)

Example: poorly dressed man

Adjectival compounds consisting of adverbs not ending in -ly followed by participles or adjectives may be open in any position if ambiguity is unlikely. If ambiguity is likely, however, they should be hyphenated.

Example: sweet smelling flower vs. late-blooming ingenue