I'm using the latest version of the moment-timezone library which claims to be using the 2023c version of the tzdb. When I look through the source code, there's a JSON file which seems to have the timezone definitions. Here's what I see for Mexico City for example. It looks like CDT is in there.
Well yes, I'd expect that - because it has used CDT in the past. The time zone database isn't purely about the future.
When I use this library and try to simulate Mexico City in my browser it displays CDT where I expect to see CST.
Please show us that code so we can reproduce it. For example, you might be displaying an old date where CDT was in use.
Jon