The slave is given a grace period for connectivity (I believe it is 24 hours). If the slave cannot connect in that time, it will disable search on its node until the problem is fixed, but keep indexing data.
I did some more searching and according to this page:
If a slave is orphaned, it generates a warning which counts against your 5 allowed violations in a given 30 day period. So it seems that search won't be disabled immediately. What's not clear is if that warning clears when the slave re-connects (in other words, does the warning count get reset when the slave re-connects
Is this still true with a "no fault" license? I would think it may be, since the license, including the part about being a no fault license, lies on the License Master, so if the IDX node couldn't communicate with it, it would still eventually give up the ghost and prevent you from searching.
For whatever reason, or SD/LM has connectivity issues from time to time, and it will show up in the splunkd.log file. It never lasts more than a few hours, and we do get messages in the web console about it, but up till now it hasn't stopped our searches.