The algorithms used at VirusTotal are unfairly biased toward automatically flagging any installer built with NSIS (because some viruses masquerade as NSIS installers). Notepad++ is not going to stop using the NSIS installer, so that means that VirusTotal will likely continue to false-flag it.
Per these results, there isn’t currently a false-flag on the official v8.5.3 64bit installer: https://www.virustotal.com/gui/url/5ed5d73a10561c63301ed2f56dd62402b0748880c93a03b147ed09de0864a783
So if you download it from the URL that the report scanned (https://github.com/notepad-plus-plus/notepad-plus-plus/releases/download/v8.5.3/npp.8.5.3.Installer.x64.exe – which is the official download URL for v8.5.3), then it’s safe.