At SXSW Jim Lee says they always planned to bring the trunks back since they removed them in the new 52: https://www.bleedingcool.com/2018/03...-jim-lee-sxsw/
"Jim Lee: It is a heavy responsibility (laughs) changing the trunks. Even when we took away the trunks, we knew they were coming back. No one remains dead, no trunks remain unworn. We knew they would eventually came back. But Brian coming over and hitting #1000 … there are things Brian is doing that will really change fans’ understanding of the character. It will make a more palpable change by bringing the red trunks back. It’s a fun thing to do, things we can play around with but, at the end of the day, it’s all about the story. As much as they react to the trunks, it’s really all about the story and the character. We have some really exciting things plan and the trunks bring notice to that."