According to Paul Thurrott, https://www.thurrott.com/xbox/194950/john-romero-to-release-free-new-levels-for-original-doom, [thurrott.com] there will be 9 new single players levels and 9 new death match levels released for free but you will need the original DOS game in order to play them. It's planned for mid February so comfortably misses the 25 year anniversary.
"SIGIL is the spiritual successor to the fourth episode of DOOM, and picks up where the original left off.”
I'm guessing you could get a legitimate copy from Good Old Games or fire up DOSBOX if you still have a version on floppy that will actually load.