When you say you saw your old plugins there, did you see them directly (as DLL files) or did you see them as folders (with the names of the plugins)?

DLL files -> pre 7.6
folders -> 7.6 onwards

At least that is how it should be.

Pythonscript install instructions for 7.6.x are here: https://notepad-plus-plus.org/community/topic/17256/guide-how-to-install-the-pythonscript-plugin-on-notepad-7-6-3-7-6-4-and-above

yes, this is the folder structure change i was mentioning above.

all plugins are now stored at a separate sub-folder with the same name as the plugin.dll, but without adding the .dll extension to the folder name.
this was done to have a cleaner structure, instead of all files mixed together at the plugins folder.

here’s an example migrating your existing pythonscript:

• open your plugins folder in explorer, e.g. using plugins > open plugins folder...
• create a new folder called PythonScript (in your case it should be already present)
• move your existing PythonScript.dll into the PythonScript folder

note: if your pythonscript version is below version 1.3.0, you can update it manually with this guide:
>>> Guide: How to install the PythonScript plugin on Notepad++ 7.6.3, 7.6.4 and above <<<.

yes, due to ongoing changes in the plugins folder structure, it is currently better not to force users to any version above 7.5.9, as they would have to relocate their current plugins, in order to use them.

also the development of the plugins folder structure is still in transition of changes, and it is recommended, that users, except power users or pros that regularly read the notepad++ community forum, wait at least for the upcoming 7.6.3 release.

note: power users can update easily by downloading the current 7.6.2 installer and installing it over their current version and adapt their plugins, but they will have to relocate their plugins again, when 7.6.3 is released.