Go to 'Negotiations' and then to 'Advanced Search' - in 'Advanced Search' there is an option for 'List' status - from there you select 'Transfer Listed'.
Amongst the Transfer Listed players who are still in contract, there will be all the other players who have put their contract on hold.
Basically, in my experience, any player that puts their 'negotiations on hold' automatically makes the 'Transfer Listed' filter.
Search for 'Transfer Listed' players, especially in January to identify the players who have 'put negotiations on hold'
Not the most ideal way to go about it - but it definitely captures the free transfer players in my experience.