The film lies in the U.S. public domain due to the omission of a copyright notice. There have been several lawsuits relating to the film's copyright status over the years and they are outlined below:

1. As explained at Debbie_Does_Dallas#Copyright, Debbie Does Dallas was exhibited without a copyright notice. After the producers failed to fix the problem a court ruling in 1987 found that the omission of the copyright notice 'resulted in the film being irretrievably injected into the public domain "several months" later'. No court ruling since then has overturned this judgment. See http://www.leagle.com/decision/19871111657FSupp454_11021 for the ruling and http://www.lawlawlandblog.com/2011/11/lessons_learned_as_a_lawsuit_o.html for analysis of the judgment. Even if the ruling were to be legally challenged and overturned at some point (doubtful considering the law is clear cut about copyright notices) then Wikimedia has a legitimate defence against a claim for damages in that a court had ruled the film was in the public domain. Neither does Wikimedia have a moral duty to protect VCX's revenue stream from the film since VCX was the plaintiff in the court case and instrumental in having the film ruled in the public domain so they would not have to pay royalties to the producers.
2. In 2010, a lawyer (Evan Stone) subpoenaed the identities of 113 John Does who had been filesharing Debbie Does Dallas (see https://www.techdirt.com/articles/20100902/11385710880.shtml). This was a classic copyright trolling case; it works like this: the lawyer identifies the filesharers (usually for porn) and offers them a settlement (usually a couple of thousand bucks). The ruse is highly effective and many of the targets pay up because the filesharers fear being publicly humiliated in a court case. In this case the court did not give permission for the subpoeneas and the judge fined Stone $10,000 (see https://www.techdirt.com/articles/20110911/01030715892/copyright-troll-evan-stone-sanctioned-more-than-10k-sending-subpoenas-when-court-said-to-wait.shtml). The filesharing case did not progress and it died in 2012.
3. In 2011 Arrow (the distributor of Deep Throat) sued VCX (the distributor of Debbie Does Dallas) for distributing Deep Throat. Arrow and VCX settled and acknowledged each others' "copyright claim" to prevent the case from going to court. As Law Law Land Blog explains, the copyright over Deep Throat is unclear; the blog states that Arrow have a very good argument that Deep Throat is still under copyright, and thus the settlement was not a good one for them because Arrow had the legal right to distribute Debbie Does Dallas. The reason they settled was because they did not want to risk the judgment over Deep Throat going against them, so they acknowledged VCX's bogus copyright claim.
4. In 1979 the Dallas Cowboys Cheerleaders successfully obtained an injunction against the Pussycat Theater to prevent them from promoting the film, on the basis that the cheerleader costumes constituted trademark infringement (see http://irenebrination.typepad.com/irenebrination_notes_on_a/2017/03/star-varsity.html). Wikimedia is permitted to host trademarks per Commons:Non-copyright restrictions#Trademark law.