For XZ-related topics use xz at tukaani dot org. It is forwarded to Lasse Collin and Jia Tan. Jia became a co-maintainer for the XZ projects in 2022. The GitHub page of XZ Utils may be used for bug reports and such too.
For other questions, Lasse Collin's email address is lasse dot collin at tukaani dot org. Sometimes I'm a bit slow to answer, but if you don't get a reply within two weeks, assume that the email has got lost, and try either resending or use IRC.
The project's IRC channel is #tukaani on Libera Chat. The channel is usually very quiet, so just ask your question and someone might wake up.
Lasse's nick is Larhzu. I'm not on IRC every day although my IRC client tends to be online. You have a higher chance to talk to me if you are on the channel 14:00Z/20:00Z (that is, between 14:00 and 20:00 UTC). If you don't get a reply on IRC in a day or two, try sending an email instead.