The good old days! One hierarchy was alt.sex.  Under that was
alt.sex.erotica.marketplace - General products/services for sale.
alt.sex.telephone - Phone sex ads.
alt.sex.bbs.ads - BBS ads.
alt.sex.magazines - Magazines for sale and wanted.
alt.sex.wanted - Looking for love or just a little relaxation.
alt.sex.services (a.s.s)
a.s.b (brothels)
a.s.p (p is what you think it is)
alt.sex.strip-clubs (a.s.s-c)
.
Under the rec (recreation) hierarchy was rec.arts.movies.erotica (r.a.m.e; RAME).  The porn stars Brandy Alexandre and Steve Holmes were regular posters there.

As I recall, usenet posts were stored on multiple hosts all over the place, but all legitimately owned and maintained.  Even if you could bring back usenet-like services, I think the liability issue would still be a problem under FOSTA SESTA.