1. Put all of the files you want to combine in the same folder. Easiest would be at root-level.
2. Run a command prompt
3. CD to the folder in Step 1
4. Enter command copy *.csv filename.csv where “filename” is the desired output filename
5. Hit enter
6. File ‘filename.csv’ is created in the same folder

Credit: https://medium.com/analytics-vidhya/how-to-merge-multiple-csv-files-in-to-one-file-5eb6ef8e579b

I think you might be missing the point. The point is the plugin is gone and people use it.

No, I didn’t miss the point; not really.
But there is no guarantee that plugins continue to get support, updates, or continual life in the form of sitting out on some website.

Having never heard of this plugin before, and it not being in Plugins Admin, is not a great sign for it.

So, if we know what the plugin did, or what the current need is, we can come up with alternatives.

Combine Plugin URL linked here gives a 404

The domain that was hosting the Combine plugin (and several others) disappeared from the web a few weeks ago — even before NameCheap decided to evict all their Russian-owned domains.

Domain name: scout-soft.com
Registry Domain ID: 1376805404_DOMAIN_COM-VRSN
Registrar WHOIS Server: whois.namecheap.com
Registrar URL: http://www.namecheap.com
Updated Date: 2018-01-11T03:17:47.00Z
Creation Date: 2008-01-11T12:42:28.00Z
Registrar Registration Expiration Date: 2024-01-11T12:42:28.00Z
# . . . 

It was a simple concatenator that combined files in the order they were opened. The question wasn’t about how it worked or the specification though. The OP wasn’t asking for help on writing a new one, or other ways to do it. They were posting that it is missing.

What’s the specification for how you would combine multiple files?

I mean, I understand the general concept, but what I’m asking is, how would you specify the files to combine and in what ordering?