"People" was not to be preceded by a number, as in "Fewer than 30 people showed up"

The reason is the etymology of “people”: it was not originally a plural noun form. It comes from Latin populus and French peuple, which are both singular in form, and generally singular in agreement (Latin sometimes has notional agreement, but I don’t know if that is attested with populus.)

The usage of the singular form people with plural agreement in English developed from a kind of “collective noun” usage, and since the singular form of a collective noun cannot be used as a countable plural form (we can say "the group are" or "the government are", but not "5 group are" or "5 government are"), it apparently used to be thought inappropriate to use people with a numeral. (Collective nouns may be pluralized to form plural count nouns like groups or governments, but that is irrelevant, as your question is about 30 people not 30 peoples).

However, present-day usage clearly does not restrict the use of people in this way. It can be used as the countable suppletive plural of person except for in certain special and uncommon senses, such as grammatical persons.