Changed old gamertag to the new system yesterday but instead of showing a # between the...

My sons changed their gamertags over the weekend, and I was pretty sure there was some mention of this on the system stating not all games would support the # and that those games would just display the new gamertags without it

That's a new thing and most non-native-Xbox-console things don't display it as a number subscript yet. Most games, back-compat, etc., will still show the "true" gamertag, which has the numbers as part of the string. The developers of MCC would have to
update to read the new metadata and almost no games have picked it up the change yet. Support will grow over time.

Hello, that feature will only be added to games in 2020. Your gamertag does contain the # but it won't display in games for now. Also it isnt confirmed whether old games will display the hashtag, all new games starting sometimes next year will display
it for sure

Ah I see. Guess I'll just wait and hope that MCC gets updated. Thanks for the help.

Not all games have been updated with the new system. So you will see that in some games.

https://support.xbox.com/my-account/manage-gamertag-and-profile/gamertag-updates-faq

Does the suffix always appear next to my gamertag?

If you’re assigned a suffix, the # symbol and suffix appear everywhere except in friends lists. You don’t see suffixes of friends in your friends list and they don’t see yours in their friends lists. Your suffix
is used in games so that everybody knows who’s who.

When will games start supporting the new format?

A few games released this year might support the new format, but most games won’t support it until 2020 and beyond. It may take a few years to migrate all new games over to the new gamertag format.