Why do so few games support the modern gamertag system?

I changed my gamertag to something that was already taken, and I was under the impression that in games from before the change my gamertag would appear like "name1234" but going forward it would look like "name#1234". I was fine with the latter since it was clear the numbers weren't part of my name, but years later my name still appears like the former in most games I play. Even first party games aren't supporting it. Strangely, the flights for Halo Infinite supported it and even hid the numbers, but when the the game came out it displayed my gamertag the old way. Is there an issue preventing games from using the new gamertags?

:)