A key word in this discussion is "unsolicited". I can see how
un-asked for criticism can be hurtful and is probably inappropriate in a singaround, whereas it might be appropriate when reviewing a professional singer.
I do maintain that criticism is essential to improve performance. It struck me when out trio started learning a new song the other day. we play a rough version and record it. we then consider how that might be improved,e.g. get in tune, firm up intro, musical breaks, speed rhythm etc. Then we play and record it again and repeat the process etc. etc. until we reach a point where we are all happy.
I am sure this process is familiar to many here. As a solo singer I guess the thing to do would be to work with someone you trust when working on material, get the feedback and improve it to the point where the likelihood of unsolicited criticism is significantly diminished.
john