I think you mean in a highly inflected language.
Because French, for instance, is also an inflected language (a lot of conjugations, plurals, etc...), It's called a "moderated inflected language" by linguists, ( a "moderated inflected language" can be considered as "synthetic")
It's not inflected enough to be able to drop the pronouns, but even when a verbal form is specific to a person, and there can't be any ambiguity, French is not a pronoun-dropper.
So the rule Subject pronouns are completely unnecessary in an inflected language is right in Spanish, Latin, Portuguese... but not in French (It's an exception among Romance languages)
The difference is in its conjugation.
esse (infinitive form) — irregular verb meaning "to be"
1st Person Singular: sum (I am)
2nd Person Singular: es (you are)
3rd Person Singular: est (he/she/it is)
1st Person Plural: sumus (we are)
2nd Person Plural: estis (you are)
3rd Person Plural: sunt (they are)