Pioneer Masters is a Magic Arena-specific reprint set that may be released in the future. It will be combining cards from all the sets currently legal in Pioneer, pulling them together into a single set for both Limited and Constructed play. The set was originally planned for release at the end of 2020, but pushed back to 2021 for more playtesting. In July 2021, it was put on hold.
It was stated that Wizards of the Coast's schedule didn't have room to focus on growing a unique identity for the Historic format while fully supporting Pioneer at the same time. Some steps toward Pioneer are expected to come as part of Historic's growth, but they have paused work toward Pioneer Masters sets for the time being.