Drat.

NetHack 5.0 changes thousands and upon thousands of things from the previous release, 3.6.7, which was 3 years ago. 17 years ago is an eternity in this game’s history. The versions may not have gone up hardly at all in that time, but the fix logs are enormous.

Adding up the line counts of the fix logs for the 3.6.X releases with 5.0, I get a total of 6814 lines. That’s bug fixes only. There’s a similarly large number of gameplay changes!

All that is to say, migrating your old save file through all of those changes would’ve been a ton of extra work to support. I know Dungeon Crawl Stone Soup can migrate old save files but they have a very carefully designed system for sunsetting removed features in a way that old save files can still use them.