Yes, indeed deleted after an upgrade!
I have in nfsexport still an old version of FreeBSD 11.4
They are documents that were always there, i spite of being partly obsolete, but usable.
For this reason I trusted that they would not disappear.
Now, developers obsoleted troff, and with it, deleted the documents, instead of at least leaving them compiled.
I have the impression that these developers are nerds without empathy and feeling for others.