I could have sworn that I've seen rules somewhere that a character can trade out a feat for a different one once they hit a level that grants an ASI, but I can't seem to find it anywhere.  Does anyone know what book this rule is in or am I just mistaken about it existing?

If it’s anywhere it’s gonna be in Tasha’s Cauldron.

Not seeing it in TCoE; it does mention swapping out skills at ASI levels as an option, but nothing for feats.

Page 4 or 6 of the PHB or DMG: DM can allow it.

That's the only rule present in 5e. 

Tasha's has rules for switching subclasses.  Much like that "system" I think a Feat swap should simply come to a DM approved/negotiated plausibly expenditure of time and coin or divine/magical intervention to make the trade (I sometimes will hand wave undertutilized spell selections and skills, sorta retconning "player never used it, so let's say character never had it, but instead always had or discovered this capacity").

Otherwise, like others I don't think I've ever seen "rules" on swapping feats, but narratively I could see someone abandoning something and developing another.  Let's also remember there are not hard and fast real rules on how many feats a character may be awarded.  The ASI substitution and the starter feat aside from certain species are guidelines for what has been described as an optional system.  Feats are definitely one of those things a DM can use to fine tool their game to the power level and style they want to run at their table. 

That does happen between systems 

A bunch of other retraining options were added in Tasha's, but it's never been added for feats. You can always go the BG3 route and allow complete retcons.

Quote from Erik_Soong >>

I am pretty sure Pathfinder 2e has the ability to retrain feats. Perhaps those who have dabbled in other systems are misremembering and associating the option with D&D 5e?

 My group has been playing with a lot of different game systems this year.  Getting some rules crossed was probably bound to happen.