I've been using unRAID for years now, and it only occured to me, is there a way to use more than two parity drives so as to have the redundancy for more than two drive failures?

No

Is there any particular reason for this? I would think you'd want to have a good amount of redundancy for large arrays. And is there anything in the works that anyone knows of for increasing the number of parity drives you can have?

I suppose the reason is, the miniscule amount of added security is nowhere near worth the amount of effort to program it.

 

The chances of more than two drives failing before you can rebuild is VERY small if you properly monitor and maintain your server. With notifications, you will receive warnings when key SMART metrics are incremented, and can take immediate action to evaluate the situation and replace the failing drive if necessary.

 

Unraid has a long track record, and in my experience here the VAST majority of data loss is caused by user error, not drive failure. Your resources are much better put toward keeping an independent backup of critical files, that way you can recover from all kinds of data loss, not just drive failure.