In the 2017 laws, the only requirement in Law 19.6 is "The player must not step into the field of play when the ball is thrown". However Law 6.B.5 (d) Exception 1 says: "When the player throwing in puts any part of either foot in the field of play the touch judge or assistant referee keeps the flag up."
This was never enforced - all throwers stand with feet crossing the line.
I was therefore pleased to see that in 2018, Law 18.22 says: "The player throwing in the ball stands on the mark of touch with both feet outside the field of play . The player may not step into the field of play until the ball has been thrown."
Standing with a foot on the touchline means the foot (and player) is out of play. Excellent! Unfortunately Law 6.26 c. still says: "When the ball is thrown in, the assistant referee or touch judge lowers the flag, with the following exceptions:
i. when the player throwing in puts any part of either foot in the field of play."
Personally I shall continue to regard "both feet on the touchline" as legal even if the toes do overlap it.
Gloucester & District Referees Society
Thanks for raising this Peter.
The Rugby Ref is in full agreement. Feet on the line being ok would seem to be a pragmatic position to take. There are bigger fish to fry and more important battles to fight than stopping play for this.