The Robins chief has also allowed youngsters Will Oakes and Elliot Wallis to play move out on loan in 2020.
The East Yorkshire club has made eight new signings after narrowly avoiding relegation - Dean Hadley, Jez Litten, Shaun Kenny-Dowall, Jordan Abdull, Ethan Ryan, Harvey Livett, Matty Gee and Greg Minikin all joining the club.
They have released a number of players with Craig Hall a surprise exit, Rovers confirming today that he will play on loan at Featherstone Rovers next term.
But Smith followed up by announcing that Keinhorst - the centre/second-row who had a positive first season at Craven Park from Leeds Rhinos - is also free to move on.
The Grand Final winner, 29, is only one year into a three-year deal with the Robins.
“Recruitment is still ongoing, and we have announced eight signings so far with more to follow in the coming days and weeks, which is really exciting,” said Smith.
“There’s a real blend of hungry young talent together with vastly experienced players that are only going to add to the squad for 2020 and beyond.
“However, there have been some tough decisions, following difficult conversations, that have been made over the last couple of days.
“Currently a few our players, namely Will Oakes, Elliot Wallis and Jimmy Keinhorst, have been granted the opportunity to go out and seek first-team rugby for the 2020 campaign, elsewhere, via year-long loans.
“These decisions haven’t been taken lightly but, due to the make-up and the progress of player retention and recruitment, we felt that their opportunities were better served somewhere else. Doing this should increase their chances to play in the first team.
“More specifically the younger players within that group, that need to be playing first-team rugby, will get an opportunity, with those chances being limited here next year. That’s not to say those younger players won’t be back here, but for now they need regular first-grade game-time.
“We wish everybody well and we’ll keep an eye on each of their games, but the decision was made for them to maximise their opportunity of first-team rugby and to allow us to move forward."