The FileSwitcher you referenced has not been updated since 2009. It’s missed quite a few API changes required to be able to work with v8.3.3. Now, not every plugin uses every aspect of the API, so it might not use any incompatible calls: So if you have 32-bit Notepad++ v8.3.3, and if you download the UNI (unicode) version of the plugin, and if you put the plugin’s DLL in the correct subdirectory (c:\program files (x86)\Notepad++\plugins\FileSwitcher\FileSwitcher.dll , assuming a standard 32 bit installation), it might work. (Oh, it actually appears to be available through the Plugins Admin in the 32bit Notepad++, so it’s easier to get… and the most recent v8.3 API changes don’t affect 32bit, so it’s a good chance that it will still work for you on 32bit Notepad++)

But unless someone has un-officially forked it, there isn’t a modern/maintained version of FileSwitcher for you to download, or a 64bit version. I did find a couple forks by trusted users on GitHub, but none of them have published updated binaries. @chcg has a version that I cannot see has any changes relative to the original, so it probably wouldn’t help. @Predelnik has a version here which claims to have fixes so that it can be compiled 64bit… but again, doesn’t share any binaries, so you would have to be willing to download and compile from source yourself.

But if you are willing to use Notepad++ 32-bit, you should be able to use that plugin.