Some long-practiced customs grow out with fear. They can find comfort in following superstition-based on customs. 請問為什麼破折號之後是ed 然後“迷信”為什麼不用s?

They can find comfort in following superstition-based (on X) customs.
( "on" should be a typo.)

They = refer to some people
to follow customs = 隨俗
-based = 以…為主的 (複合形容詞的後半部)
superstition-based customs = 以迷信為主的習俗

Your questions:
1. the second part of a compound adjective 複合形容詞 could be an adjective, a present participle (v+ing), a past participle (v+ed) or a noun.
e.g. long-practiced (形容詞-過去分詞)
e.g. superstition-based (名詞-形容詞)

2. if the first part (also other parts) of a compound adjective is a noun, it must be in singular form.
e.g. man-eating bear (not men-eating bear)
e.g. a three-page letter, a three-year-old child
(each component of a compound adjective is considered as a noun modifier and modifiers have no plural form. 所以”迷信”不用加s!)

句子的結構拆解其實是

They can find comfort in following superstition, which is based on customs.

意思是這些迷信supersition (或從你的角度認為是指眾多supersitions)其實是「由習俗造成的」。

至於在這裏用"破折號"(希望原文是破折號而不是"-"hyphen)取代 ", which is" 是否適合，小弟的意見是不太合適：由於前句的long-practiced是以這類寫法，會令人更易將後句的寫法看成為"superstition-based這樣的組合，然後後加上on customs便句子結構混亂。但實在不是。假若真要為"省字"或"空間"而寫，建議把那" - "隔開成：
They can find comfort in following superstition - based on customs.