I can't really answer.
Nevertheless, I don't think it's a Win7 / Win10 problem. It's more a problem with your gcc installation(s).
On your Win10, this dll is probably found somewhere in the Windows path, but probably not in your desired compiler path. As I told you in a previous post, I see several gcc install in your path. So, may be (once again, I can only suppose ...), this dll is in one of this install on Win10, but not on your Win7 PC.
I made a google search with liblto_plugin and found :
https://stackoverflow.com/questions/11248116/using-mingw-to-compile-c-code-but-error-liblto-plugin-0-dll-not-foundBut you can find other solutions.
As you told, you have only some .o (and probably also .mod) for some part of your soft. They were compiled with gfortran 8.3, from a gcc 8.3 distribution. OK. But are you sure you use exactly the same compiler from the same distribution ? I say that because some distributions are statically compiled, so no need for dll in general, and other are dynamically linked so need dlls.
So, if your .o were compiled with a dynaically linked compiler and you use a statically one, this is the problem... But, just a supposition.
An other supposition : some installers on some distributions require to set a specific environment variable (via an install.bat or update.bat ...). may be your Win7 and Win10 install have not the same environment variables ...
After looking in my own gcc install, I can find this dll. So I can suppose than on your Win7 PC, the path to this dll is wrongly configured. Why, I don't know. If you have it, try to add the folder where it is located in your Windows path (or with a -L linker setting, search path for linker).