A tough question. As you note the side bar on pg 216 is a little ambiguous if ordinary nobles can buy/carry ordinary flintlocks. I am no one official. But I think its clear from the flavor text that flintlocks are more restricted in 5e. There is also this small note on the bottom of the weapons table.
5eACG Weapons Table pg 212-213 wrote:
†Commonly only Val or Heroes from the Republic of Altheria may buy and use legaly use flintlocks [sic].
From this I would *guess* that you cannot just buy a flintlock unless your PC falls into one of three groups: (A) noble vals or (B) Altherians or (C) clergy of Altheres. I would also assume that some powerful (NPC) nobles still have or use them. And I might
guess that its possible through campaign play for a highborn PC to gain access to purchasing non-adorn flintlocks.
TMK this is different from the A:RPG campaign (where non-val nobles can buy non-adorned flintlocks). Again
guessing...the systems are sufficiently different in how they handle the mechanics of the weapon, that different rules of access may be in place. In A:RPG flintlocks are astronomically (100x) more expensive than similar weapons and essentially worthless with out maxed out ranks in the skill Marksmanship which already limits a lot of players (you also want to be at least proficient with them...) That in itself guarantees the campaign isn't awash in them. By comparison in 5e they are simple weapons that only cost 4-8x as much as a sword. So some other restriction on their availability/desirability may make sense.