Parsing a date and timestamp to default Elasticsearch

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The date filter does not parse named zone, you should change your match to the one shared in the previous answer.

date { match => ["event_timestamp", "EEE MMM dd HH:mm:ss 'IST' yyyy"] }

Also, you will need to specify your timezone to the filter and you can't use named timezone as well because they can be ambigous, in your case, IST can mean 3 completely different timezones, as explained in this another question

You will need something like this:

date { 
    match => ["event_timestamp", "EEE MMM dd HH:mm:ss 'IST' yyyy"] 
    timezone => "Asia/Kolkata"
}

Assuming that by IST, you mean India Standard Time, so you should use Asia/Kolkata as the timezone.