Hello! Thank you for the responses. The general advise I've received in the past is that 404's don't harm anything. However, my findings have been very different.

The 404s arise from the fact that the user profiles were deleted. I'll try to restore them.

I'm working on restoring SEO for an old forum that has over 300,000 deleted profiles. I checked that all those profile pages are throwing 404.

I can setup route to take care of these old profile pages and I'm wondering if I should setup a page that says the profile has been deleted? That way, at least Google won't report 404.

Or should I leave it as it is? Google's reporting 404 since last 4-5 years on this site. Let me know your thoughts.