The logstash-input-sfdc_elf plugin is not made by Elastic, it is a third party plugin.
Besides that, this plugin seems to be abandoned, it was last updated 8 years ago, a lot of things changed in Logstash in this time and I'm not sure this will work with recent versions.
You can find the code here, you will need to contact the developer for support.
You will have to check in the Sales Force documentation on how you can access this log, depending if you can read a file, use a API to get the logs you have a couple of inputs that you could use.
I do not use Salesforce so I can not provide more information about it, but check the Sales Force documentation that will give you some hint on how to proceed.
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