You are actually in the incorrect forum for this question. I believe you might get a more accurate and "confirmed" response in
Oracle Database - General Questions: General Questions
None-the-less, I took a look at the certification matrix for Oracle Database Client 10.2.0.5. It does show that it is certified with Oracle Database 18.104.22.168.0.
If you have access to My Oracle Support, you can check the certification matrix out for yourself.
As far as your questions go regarding known issues ... the suggested forum might help.
Hope that helps some.
Is a 10g client (10gR2) able to connect to a 12c server? Is there known issues?
This seems to be a simple application (a bit of PLSQL, one schema, no fancy feature). If we decide to try, I'm planning to dump the schema and reimport it into the 12c, and chang only the TNSNAMES.ORA of the client.
Why wouldn't you just use a 12c client and eliminate any problem with connectivity between versions?
Just use the 12c client after you import into 12c.
The 10.2.0 client to 12c has limited extended support.
Also the 10g end MUST be at 10.2.0.2 (or higher) in order to use PLSQL between those versions. See Note:4511371.8 for more details.