Gloucestershire-born Thomas, who joined in the summer of 2019, has struggled to establish himself in the Reds' line-up during his time with the club.
The 22-year-old, whose Oakwell deal runs until June, 2023, made 19 league appearances for the Oakwell outfit last term, with 12 arriving from the bench, before being allowed on loan to League One side Ipswich Town in January.
Thomas spoke about losing confidence during the season with Barnsley and being desperate to get out and play games at Ipswich.
The wingman played just five times for the Portman Road side, with the last appearance being in mid-February.
He later revealed that he needed to take a break from football for the benefit of his mental and physical health.
In a statement released on Twitter, Thomas wrote: "I've received quite a few questions over my lack of involvement for both Barnsley and Ipswich in recent weeks and would like to address the situation.
"For my mental and physical health I needed to take a few weeks away from the game that I love.
"I am now delighted to say I am getting better each day, feeling rejuvenated and can't wait to get back to my very best next season and do the fans and the club proud.
"Good luck for the rest of the season. Thank you to everyone for their love and support."
Barnsley brought in Thomas from Derby County in the summer of 2019 - having been impressed by his successful loan spell at Coventry City in League One in 2018-19.
The Gloucestershire-born player scored five times in 44 appearances for the Sky Blues, including a fine strike in the 2-2 draw at Oakwell in the spring of 2019.