Software vendors are testing for backward compatibility (and support).
However, what you want is a 'kind of forward compatibility support'.
Otherwise the number of combinations to test would be endless.
I am afraid but obviously due to that reason you will never receive such statement.
A lot of intelligence is built in the NW Client software.
If you only use the NW 7.6.3 functionality of your NW 8.1 client you should not have any issue. AFAIK, this is by design. And the logic behind it is pretty clear.
The only issue is - how do you find out where the limitations are? There is no such doc.
So the only way to do this is by trial & error.
If you run a server-initiated process, you should be pretty save. Why? - because the Server can only initiate what he has built-in - he does not even know about new client SW capabilities.
However, if you start a client-initiated process you might end up using a command option which the older NW server software does neither know yet nor he can understand it. The consequence is that he 'cannot execute (properly)'. This might result in unpredictable effects like
- No response at all
- Error messages (if you are lucky you receive something like 'not supported' or 'wrong arguments')