There may be some more information in the meeting notes: LLVM Embedded Toolchains Working Group sync up - #62 by voltur01
One proposal was that if lld had a JSON output format for the map file then the existing lld map file could switch to GNU ld format, with a conversion script from JSON to “old” lld format for those that preferred/relied the old format. I expect retaining the existing lld format wouldn’t be too onerous, but may help with maintenance in the long term.