Screeenrant can now confirm that the title has been indeed changed to simply "Supergirl."
Speaking with Rolling Stone, James Gunn revealed that Milly Alcock's DCU movie will just be called Supergirl. Gunn remembered how he was originally going to call his Superman reboot Superman: Legacy before deciding to shorten the title to just the hero's name. While he did not directly state why he changed the Supergirl title, he mentions that he is "sick of the superhero title, colon, other-name thing." His experience with Superman certainly seems to have impacted DC's upcoming Supergirl movie.
Somehow Gunn may have forgotten the 1984 Helen Slater film with that title. Of course, it was a bit of a bomb at the box office.
[A bit OT, the last few days I was unable to log on to the site. Not sure if it's my connection or something on the servers here but I could not log on here.]