It’s clear from your question that you considered that sentence to be in the past perfect tense, so it should have used the verb form “had developed” instead of “developed” to read as follows: “The communist legacy of isolation and the consequent stereotypes that had developed also exerted a huge influence on the self-perception of people with disabilities.”
Using the past perfect form “had developed” in that sentence is incorrect, however. This is because the sentence you quoted isn’t in the present perfect but in the simple past tense. The long noun phrase “the communist legacy of isolation and the consequent stereotypes that developed” is its subject, “also exerted” is the operative verb, and the noun phrase “a huge influence on the self-perception of people with disabilities” is the sentence complement.
Indeed, in that sentence, the word “developed” isn’t functioning as a verb. Together with the conjunction “that,” it forms the descriptor “that developed” to modify the subject, “the communist legacy of isolation and the consequent stereotypes.” That descriptor—it doesn’t use the auxiliary verb “has” or “have” or “had”—simply reports that the development took place without indicating whether it has ended, is ending, or will end sometime in the future.
So when would “had” be needed to work with “developed” in a sentence? It’s when we need to use the true perfect tenses to indicate the completion or “perfection” of that development in relation to a particular event or point in time.
We use the past perfect tense (had + past participle of the verb) when that development was completed with respect to another action or event in the past, as in “The communist legacy of isolation had developed before the country could institute democratic reforms.”
We use the present perfect tense (have + past participle of the verb) when that development is completed with respect to the present, but precisely when isn’t specified: “The communist legacy of isolation has developed because of the weakness of the country’s democratic institutions.”
And we use the future perfect tense (will have + past participle of the verb) when that development will be completed with respect to another future action or event: “The communist legacy of isolation will have developed by the time the country’s dictatorship decides to stop its brutal expansionist tendencies.”
