I think any module should assume that the 4 "classic" classes are covered, namely cleric, wizard, fighter and thief.
Ugh, no. This expectation that you have to cover all four classes is one of the things I've always hated about D&D. I can't count the number of times I sighed and chose not to play the character I wanted to play, so the group would have a healer, or a front-line fighter, or whatever. 4E has reduced that dependency and I hope 5E eliminates it.
Even if a party has both a divine caster and an arcane caster, there's no guarantee that they have access to a given spell. They might be a favored soul and a sorceror who spent their limited slots on other things. Or the wizard might be a specialist who's barred from learning that spell. Or the wizard simply might never have gotten around to learning it.
Bad enough that the combat system expects a party to cover the "classic roles,"
but you can usually muddle through combat without one. But an adventure written on the assumption that the party will include [caster type X] with [spell type Y], and that cannot be completed if you don't, is a badly written adventure IMO.
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