Hi @imranazad
sorry to hear you are having trouble with restarting your cluster. I just tried re-creating the scenario you described locally with 6.8.3 and while I can see the synonym parsing IllegalArgumentException that you already diagnosed correctly (btw, the "lenient" parameter can also help skipping invalid synonym rules, problem there is that you don't notice theire not working until much later), the result for me is that the index in question is "closed" after restart and cannot be openend, but I don't get a red state.
Do you remember the update procedure? How many nodes do you have and did you take them all down or performed a rolling restart similar to the procedure described here (just without the version upgrade)?
The scenario I tried was just a single local node, stopped that one, added an invalid line to the synonyms file and restarted, leading to a "closed" index but no red state.
At this point I would also doubt the synonyms being the only culprit, but to be sure we'd need a bit more insight into the logs.