Allen, 28, has scored one try in 18 appearances this term and has made a total of 132 appearances for the Vikings in two spells at the club, while Butterworth leaves without having played a first team game.
Coach Denis Betts said: “I’d like to wish Dave and Kieran all the best for the future. Dave has been a good servant to the Vikings during his two spells at the Club and he leaves here at the end of our campaign with our best wishes and luck for the future.
”Kieran hasnt managed to make an appearance for the Club this year. It has been difficult for us to find him game time elsewhere due to the way the system is and he’s too old for Under 19s Rugby League. However, I know Kieran has enjoyed the full-time environment and being around the culture we have here, we believe hes benefitted from that.”