Do caster out of combat options matter?

[Vahnavoi]

But the thing is, it doesn't make sense if "one of your teammates needs to have the right spell" is the ONLY way to progress toward a goal.

Whether it makes sense is entirely situational. The actual important part is that whether players have just one or multiple good choices to pursue a goal, they can also blow those choices.

If the campaign involves the bad guys using an underwater temple, and none of the group's casters is able to cast Water Breathing, is the DM supposed to just go "well dang, guess you can't go deal with that temple."?

Despite the fact that the DMG shows *multiple* magic items that could help with it, and the fact that the casters in the group are not likely to be the only beings with relevant magic in the world?

You are loading the question. The fact that a dungeon master can place multiple ways to approach the temple, does not mean they have to - and regardless, as noted above, even if there are multiple, the players can just blow them all. It is, in fact, perfectly fair to note to players "you did not make any of the choices that'd allow you to pursue that goal, so either call it quits or pick another goal". Though if players are even mildly self-aware, they'll make the same observation on their own once their error becomes clear. As a player, being able to evaluate when a strategy has failed, and coming up with a new strategy, are what you are supposed to do.

Handing every solution for every problem on a silver platter isn't a good thing for a campaign or for GMing in general. But refusing the idea that there are solutions beyond "what's written on the character sheet" isn't a good thing for a campaign or GMing in general either.

You are arguing against a point that wasn't being made. What I said doesn't care about whether the options players are picking from are on their character sheet or somewhere else.