Former Wolves defender John was among Rovers’ long-term injury absentees last season due to a back issue and his three-year spell at the club came to an end this summer.
The 22-year-old this week signed a two-year deal with League Two side Rochdale.
Askern-born left back Horton, 21, was also released after failing to hold down a first team shirt and has since joined National League side Chesterfield.
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McSheffrey said: “Cameron for me is a left centre back. He missed the whole season really, I think he made six appearances. It was important to get him back out there at the end of the season and get him an appearance to show everyone that he’s back fit.
"But we treated that one in terms of how we’d try and treat a new player coming in as well.
"If I brought a new player in and they’d not kicked a ball since last September you [the media] would probably be hammering me and the fans would probably be hammering me!
"So we had to look at some of the retaineds in that way and as harsh as it sounds that was really the thinking behind it.
"I’m delighted Branden’s gone to Chesterfield and that he’s got sorted. He’s a good lad, I had him in the youth team.
"Sometimes you just need that challenge in life and that change.
"You can get a little bit too comfortable at a club where it’s just easy for you to turn up every day and train and at times you can drop your levels and your standards.
"I hope this challenge and change of scenery will really get the best out of him and he has a really good career.”
Rovers have been linked with a move for former Sunderland left back/left winger Cieran Dunne.
Should no further new faces arrive at the club this week, McSheffrey may opt to field a trialist left back in this weekend’s opening pre-season friendly at Armthorpe Welfare.
New signings Harrison Biggins, George Miller and Luke Molyneux are all set to feature for Rovers for the first time on Saturday.