There is a fresh (minimal) install of CentOS Stream 10. There is no default file in the /etc directory (/etc/sasldb2). I run this command:

# pwd
/root
# saslpasswd2 -c -u example.com user
Password: 
Again (for verification):
# echo $?
0

The saslpasswd2 command created a file in the current directory with %{gdbm_db_name} filename instead of /etc/sasldb2.

# ls
anaconda-ks.cfg %{gdbm_db_name}

In this directory I run the command below

# sasldblistusers2
user@example.com: userPassword
# rpm -q cyrus-sasl-lib
cyrus-sasl-lib-2.1.28-27.el10.x86_64

I have read in the sasldblistusers2(8) man page the default file is the /etc/sasldb2.