Back in September, we announced that the F-Droid ecosystem is moving fully to YAML formats. That means the old .txt custom metadata format as well as the JSON metadata format will be entirely removed from all the tools, like fdroidserver. The upcoming 2.0 release of fdroidserver will only support .yml metadata files.

Now is the time to convert! Thanks to the hard work of @uniqx and others, it is easy:

$ fdroid rewritemeta --to yml

The .txt. and .json formats will only be available in fdroidserver 1.1.x releases from here on out.