If your personalization becomes so extreme that another fan of the setting wouldn't recognize it
I think this probably nails it. I used to use Greyhawk as "neutral ground" for one shots and the like. If you told me you were running a Greyhawk game, would most of my assumptions be applicable? If you can sum up all the meaningful changes in a paragraph or so, it's probably personalization. Probably. If it takes much more, or if my assumptions are a liability to playing (and not because I'm being a purist), it's home brew. It's definitely gray, though. I would imagine that, if you drew your own map, but pulled in Dragonmarks, kalashtar, warforged, distant gods, pulpy feel, etc., I'm probably just playing a personalized version of Eberron -- again, it depends on whether I could say "We're playing Eberron, with these tweaks" and have it be beneficial to a player new to my table, but familiar with Eberron, or if it would give them too many false assumptions. I definitely see that as being a lot less likely, but theoretically possible.
I think this is a really, really significant part of my issue with the Realms in 5E. We're heading in a general direction where I could invite a new player to my table for "D&D" and they assume that means they should select from the five factions, the SCAG is in use, etc. If I'm using Eberron or a home brew, those assumptions are going to be a liability to everyone's enjoyment of the game.
On a more amusing note, I've got a player who has (I think) read every Realms book published in the last 20 years or more. He was somewhat disappointed that SCAG wasn't a "true" 5E FRCS book. I keep threatening him that I'm going to pick up the SCAG and that will be the
only canonical source used. I'm going to drop an island off the west side of the map that's filled with kalashtar and dromites, the Harpers will be secretly controlled by a cabal of evil doppelgangers, etc. To your question, that's probably home brew, but could be considered personalized, depending on whether or not you take WotC's position of "all the material for older editions is still valid".