As with so many things, there is a spectrum.

At one end is the singer for whom knowing the song perfectly is a point of honour and who would therefore rather not sing at all than have any kind of prompt sheet. Unfortunately they may nevertheless sometimes forget the next line or the next verse: then they improvise, go "la-la", get a prompt from someone else in the room or just stop in the middle of the song.

Then there's the singer who refers to a prompt sheet (or a smart phone) if it becomes necessary.

At the other extreme is the singer who reads the words as if they've never seen them before, struggling even to make them fit the tune.

We can all have our points of view on this, but I know where I personally draw the line of acceptability.