Kick it Out says it is "concerned" Newcastle have not outlined the reasons why Peter Beardsley left the club following racism and bullying claims.
The Premier League club held a 14-month investigation during which Under-23s coach Beardsley was placed on leave.
The ex-Newcastle and England forward has "categorically denied" the claims.
Kick it Out, an anti-discrimination charity, said it would "encourage" the club to be "transparent" if Beardsley left as a result of the allegations.
Beardsley faced multiple complaints, including one of bullying by 22-year-old academy midfielder Yasin Ben El-Mhanni.
Since Beardsley left the club, the Football Association said: "The allegations made against Peter Beardsley continue to be under consideration by the FA."
Kick It Out said in a statement: "We are continuing to ascertain what the FA's investigation includes and would welcome a swift and informative outcome for all involved."
After leaving the Magpies, where he made 326 appearances as a player, Beardsley said the "time was right to seek a new challenge".
He added: "I have always honoured my contractual obligations of confidentiality to the club and maintained my silence, which has in itself been very difficult."
Newcastle said it was "grateful for the contribution Peter has given over the years as a player, coach and ambassador and we wish him well for the future".
In 2003, Beardsley and academy director Kenny Wharton faced a Premier League investigation over allegations of bullying made by youth players James Beaumont and Ross Gardner but the case was dismissed.