You use the NSP files only for installation, the "real" data is stored either on your internal memory or on SD card.
So yes, you can remove the .nsp from the SD card afterwards.
Since they ask these kinds of questions. Is there a way to delete the copies installed in the internal memory and be able to play online again without being banned by deleting the backup records?
nah, discord is also a shithole. discord is good only if you have your own community with your own rules. or if you re lucky to find some groups that arent shitholes.
as about forums, when i recently revisited some old forums i used to be regular and seeing how immature most members still are i was like "Yeap. Nothing of value has been lost.". Unfortunately it's the good, comfy forums that ended up dead or shut down, especially because of internet centralization.