To be fair, the way the secret content is "discoverable" involves a very specific sequence of choices in a single conversation that you cannot go back and re-make without undoing a lot of game progression. Old games didn't have the same kind of progression, and new games are generally more forgiving about post-game exploration of all the content in the game. The game also has misleading affordances that give the impression that your relationship with this character is built up over several conversations, when actually only this one specific sequence in one conversation is known to work.

So, I don't think it's bad because the content is hard to discover without outside knowledge, it's bad because it's both hard to discover and impossible to fix once you have progressed past that point. It's not like being able to just rewatch part of a film or re read a book chapter to get a reference. You basically have to reload an old save, go through a decision sequence, and then replay the entire last third of the game until you have access to a final playable level, and the way it's implemented feels like it was very much slapped in to take the place of a johnny-friendship-meter mechanic that appears mostly broken outside that one conversation.

The King’s Quest design aesthetic.

it sucked then, too.

But King's Quest is a lot faster to go through on a new run.

that’s a depressing thought.

I finished all main/side missions and gigs, let Johnny take over whenever he wanted and then went through all reachable ending combinations (8 different endings, even if slightly) and then I learned there is a secret ending considered the best of them all which was denied to me due to some obscure choice early. So no wonder it left a sour taste.