I'm not (just) being facetious - I don't think it can be well answered, because you're obviously thinking of something specific, and someone's going to answer with their own probably different specific 'herbal' tea in mind.
I don't know about the biological definition of "herbal" but in practice herbal is used as a catch-all for tisanes, teas that are made without the Camellia sinensis (tea) plant. It's not a hard rule, e.g. rooibos teas are generally listed separately from herbals.
In any case, there are a lot of different herbals and the perfect water temperature might be different for all of them. When I drink herbals (rarely) I just use boiling water.
Fun! Another hidden trap when English is not your native tongue, I guess. Today I learnt that "herbal" does NOT mean "with herbs", but instead means "with PLANTS". All of them! Thanks, English.
In Swedish, the word for non-tea tea is "örtte", where "ört" means "herb". Very confusing.
It depends.
In America at least people generally mean Herbal tea to mean any beverage steeped with plants that is not made with the actual tea plant. They also may or may not be including Yerba Mate in this, which is a South American beverage made with a different plant, but still caffeinated.
In other cooking context Herbs pretty much mean fresh (not dried) plants that are used to add an aromatic flavor, such as Basil or Rosemary. A chef would not refer to a carrot as an herb.
I'm not (just) being facetious - I don't think it can be well answered, because you're obviously thinking of something specific, and someone's going to answer with their own probably different specific 'herbal' tea in mind.