The Saber tag would imply that Artoria is in the picture, all I see is Mordred, yes, she's Artoria's son, but I'm not sure if that is enough reason for the Saber tag.
In Danbooru it looks like the Saber tag is used for Artoria instead of for the Saber Class so yeah, there's no Artoria here, fell free to remove the tag.
Mordred is still a clone of artoria and a member of the Saberface family. so while she isn't artoria, she still technically IS artoria and a saber none the less.
