The documentation for list=alldeletedrevisions says:
adrcontentformat: Serialization format used for adrdifftotext and expected for output of content.
But the adrdifftotext parameter is marked as deprecated, though adrcontentformat isn't. Will that happen in the future or is there other use case?
It was not deprecated with adrdifftotext because it is also used for the output format of the content, as is stated in the documentation message you quoted. But the work on multi-content revisions may deprecate it, since it doesn't seem that any content models are actually supporting multiple formats.
Resolving this since the question was answered and there's nothing left to do here.