Can an Oracle 19c database dblink to an Oracle 10.2.3 database?

Yes. I am upgrading from 11.2 to 19 and my link to 10.2.0.3 works:

SQL> select * from v$version@MYDBLINK;

BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - 64bi

SQL> select banner from v$version;

BANNER
--------------------------------------------------------------------------------
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production

I have a link to a 9.2 database and it does not work, but I have a work around.

Bobby

Based on this document from Oracle Support, the answer appears to be "No". A 19c client (the owner of your DB Link) is only supported/compatible with a server version of 11.2 or greater.

Conversely, a 10.2 database can only support a client as new as 12.1, so any upgrade of your 12.1 database to any newer version will break your db link.