PrC prerequisites based on in-game events, such as finding a magical item, or requiring a 'gatekeeper' person or organization, are absolutely terrible. They have a large potential to cause conflict between DM narrative decisions and player expectations/agency.
No PrC should ever have a prerequisite that isn't under player control: Race, Class, Backgrounds, Ability Scores, Features, Proficiencies or Feats.
I think you should still play 3e
First, notice that "prerequisites under player's control" does not eliminate conflict with the DM. See what happened in 3e: they idea
forced all DMs to straight veto a lot of prestige classes. Narrative prerequisites (which actually I think they could be extended
afterwards i.e. requiring the PC to obtain other story achievements to progress into higher levels of the prestige class, not just the first) just honestly bring the issue into the foreground, by telling everybody that prestige classes may have negative effects on the game, and thus the DM is supposed to keep an eye open when using them. With mechanical prerequisites only, all you get is to give control in the power of players, which will then abuse it and pretend it's their right to do so, make the DM feel "cheated" and force her to say NO in advance just out of prudence.
Second, mechanical prerequisites creates the need for "builds", meaning it forces players to think ahead, possibly since the start of the game. While this can be a lot of fun for some players, it is largely detrimental to other players. It worked well in 3e because the whole edition was built around the idea of
system mastery: as a player you were
supposed to "study" the system, discover the best combos, and exploit them. The idea was that such player should be
rewarded. The flip of the coin is that it makes everybody else feel
penalized for not spending hours studying the books (or money to collect all possible splatbooks, and cherrypick a prestige class here, a feat there, a spell over there...). And 5e was specifically designed among other things to also cater for
casual gamers, who simply do not have time to plan ahead what their PC should become 10 levels later... so if you make a prestige class that requires
even just one proficiency, you are screwing up all casual players who didn't know about that prestige class option before, and now can't do anything about it.
There are also other negative effects of the "build" playstyle, for example it makes a lot of players way too focused on levelling up all the time (because they won't feel their PC is ready until it reaches the last level they designed), and not concentrate much on what's going on in the actual game.