Do plugin need to be open its source

i wonder to know because ue4 opened its source to get better development so i wonder do plugin alsoopen source and are there any license under using

I would assume no. The games made by ue4 are not open source… some maybe. but triple A studios use this and don’t give anything out.

For the Marketplace you have to show the source for the Marketplace review team.
But after they review you can opt to publish without source;

I choose to always publish with source because marketplace users are developers and you never know what exactly they need and having source for them is very important in most cases.
If a plugin is closed source I personally don’t buy it.

Yes true, in general if the plugin is not open sourced it will be troublesome especially if the developer lost the interest to maintain it in future. But there are ways to circumvent around it - the developers can provide C DLL (which contain the valuable source code) but they also provide the full source for the plugin wrapper. Of which I think is fair.