You can use this link to directly download old versions of plugins:
http://assetgame.roblox.com/asset?id=....&version=....
This is intended for places but also works for plugins despite the fact that no Version History is available on the website for plugins.
For example, when I go to this link:
http://assetgame.roblox.com/asset?id=186088172&version=1
This will get the first version of the plugin you link that you uploaded:
If you rename this file and add “.rbxm” behind it, and then drag it into a Studio window, you can get the contents of that version of the plugin back in Studio:
So now the only thing left to do is figuring out the version number, but we have no list available. I would recommend starting at a high number (say 1000) and then putting that into the link for the version, and reducing it by half every time the request fails (so if version=1000 fails, then you are too high, so go to version=500, etc). Once you get something back, increase the number to halfway from where you came (so if version=500 worked, try version=750, and apply the same strategy).
FYI: That’s also known as binary search algorithm (https://en.wikipedia.org/wiki/Binary_search_algorithm)
This might take you a little bit of time, but you should be able to find the latest version number (= N) that you uploaded for this asset this way. That should be the thing you published over the original plugin, so just take version N - 1 and that should be what you overwrote.