Here is what Rabbi Yoel Lieberman says in the article present at the link
"We must immediately say, that the issue of Pilegesh is far from an issue which just has fallen out of custom, but rather one which is brought down in Shulchan Aruch who sees the option of Pilegesh as an absolute prohibition. The Rema who on the one hand quotes a more lenient position, also quotes a more severe opinion based on the Rambam, Tur, and Rosh, that this is an absolute Torah prohibition "There must not be any prostitutes among Israelite girls" (Devarim 23:18). "
However,it does not seem to me that this is the way it is.
We read in fact in Shulchan Arukh (translation into English from Sefaria site;I do not know if it is correct about the original Hebrew text) :
"A woman is not considered to be married except by way of betrothal in which the kosher betrothal was done appropriately. However, if he were to lie with her by way of harlotry, without the name of betrothal, it is nothing (towards her status as being a married woman). Even if he lies with her with the intent of marriage, mutually agreed between him and her, she is not considered as his wife and even if she dedicated herself only for him, rather the opposite is true and he must be forced (by Beis Din) to send her away from his home."
Rema:" For certainly she would be considered an embarrassment for immersion in a mikveh and he will lie with her in ritual impurity (niddah); however, if she dedicates herself exclusively for him as his wife and she immerses for him, there are those who would say that this is allowed and she would be a Pilegesh as described in the Torah and there are those who say that this is forbidden and they should both get whiplashes from the Torah as they have transgressed the precept "don't be a kedesha"
The Rema clearly explains that this passage is related to the risk of transgressing the laws on the niddah, while the case of the pilegesh, albeit controversial, does not concern this question since it is assumed that the pilegesh goes to the mikveh, being the her a public relationship.
The following passage seems to me to confirm that the passage by Even HaEzer 26: 1 has the meaning that the Rema attributes to it:
"One who had a pilegesh (commonly translated as concubine, a woman with whom one lives but does not necessarily have a full contractual, formal marriage lacking kiddushin, acquisition and engagement, or ketuba, contract of marital obligations, which of the two precisely is a dispute in the Talmud and Rishonim. The permissibility of a pilegesh is also a dispute. In this case, the pilegesh is a partner, a woman one is living with and the binding legality of the relationship between the two is in question) and it was not testified that he betrothed her (via formal kiddushin) - she is permitted to his relatives. However, if there were witnesses that the (pilegesh) woman (herself) said: "he betrothed me before two witnesses," she is forbidden to his relatives. However, if she said: "he betrothed me," simply, and she did not say: "before two witnesses," there is nothing (of relevance to court) in her words."
Here Maran points out that the halachic lawfulness of this relationship is controversial,but he recognizes as a concrete hypothesis that a Jewish man has a pilegesh ("One who had a pilegesh"),without stating that this is halachically prohibited.
Therefore, in my opinion, Shulchan Aruch cannot be said to prohibit pilegesh,since Maran leaves this question open.
However, I ask myself :is correct this English translation of Even HaEzer 15.30 present in Sefaria site? I think in primis about the passage from "commonly translated as concubine" to "between the two is in question".