Awesome replies, thank you so much, everyone.
sisoon, you have read the situation correctly. Which means, NowWhat106, that I should clarify to you that the OBH is not in touch with my WH at this point.
Roughly...
Early 90s-- The affair in question happened, after which my WH and the wife AND the OBH remained friends.
Mid-90s-- I started dating and later married my WH and was thereafter friends with this family and spent significant in-person time with them for about 10 years (without knowing the past).
Mid-00s-- ~Last time I saw the family in person, WH also saw the wife once in person a year or two after I did. WH continued to keep in touch and I sent Christmas cards, etc.
Late-00s/Early 10s?-- WH tells me that wife and OBH have divorced. From here on we don't have contact w/OBH, though contact w/OBH would have been tangential by then. My WH stays in touch w/wife somewhat via text/email/etc. I feel like at some point I Googled or WH told me or something and I think OBH might have remarried. I can ask WH.
From then on until 2022, WH continued to be in touch w/wife as her life was falling apart. There is one time in the late 2010s that I won't go into that my WH and wife saw each other in person-- long story, but I don't think anything happened there.
Now let me address your points and questions, sisoon, because they are excellent ones.
My opinion is that this is old business that's irrelevant to OBS's life today, so telling him probably isn't going to help or hurt him. That leads to a question: What positive outcome do you think will come from telling OBS that your WS is one of his XW's aps?
I'm just sort of thinking/writing through this. I'm going with feeling more than making some sort of a case.
First, I think this goes to what NowWhat106 said. While OBH and my WH have a basically lapsed relationship, OBH may still be walking around thinking that WH was his friend. Or there's a lingering doubt in OBH's mind about my WH. Or, of course, he could know about my WH.
I think as a BS I'd actually want to know the entire shape of my world. Honestly. Especially if there was that lingering doubt (which there probably almost by definition is-- unless he knows outright). I dislike the idea that "well, he probably already knows anyway..." But maybe that's a more problematic sentiment when the marriage is ongoing.
But I DO see knowing the truth as a positive outcome. At least a potentially positive one. I feel in my case that learning the truth about my entire marriage on my DDay(s) was a huge positive outcome for me, even while it hurt. It helped set me (mostly) free.
I note in myself some desire for my WH to be confronted by the fact that other people know he is "Not a Good Guy." This has been one of the cruxes of... everything. He was more concerned with his self-image than empathy, especially for me. He is already experiencing this in a couple of important ways with other people, and he is growing from it. Since he never dealt with any of these issues, he could really (largely) pretend they were sort of in some other fantasy land. I have been pointing out that certain people either knew or strongly suspected (with some proof) various infidelities, and he wanted so badly to pretend the infidelities weren't real that he was blinkered to some pretty obvious signs that others knew. And in addition to reframing the past for him, he's having to stop rely on me and my codependent and healthy support and my secret-keeping, when he actually talks to people about this other than me (his therapist, which he never had, and at least three friends now, plus his "first AP" from early in our relationship*).
I think what plays into this, too, is that I was shocked I hadn't thought of WH telling the OBH in all these many months, even after reading SI extensively, including this awesome thread I sent to WH a few days ago.
https://www.survivinginfidelity.com/topics/637329/im-still-not-doing-enough/?ap=1
In that thread, the OP is a WW, but also was the OW to her married AP when she was single. She did communicate with the OBW. WH read it and talked about it and I asked him if he had ever thought of telling OBH... He said, "no." I think that's telling. I'll get more, but I assume he meant he never even thought of telling in the 90s, when wife broke up with him.
And this isn't revenge against or punishment for my WH, but it is somewhat about my being gaslit and tired of carrying WH's shame for... basically our entire relationship, but bare minimum since he first cheated on me (another incident) a year into our serious dating relationship and I stayed and didn't really tell anyone.
IDK whether you should or should not tell OBS at this point, but if your primary motivation is to assuage some guilt you feel, you're doing it for yourself, not for him. If I were in that sort of spot, I'd want anything I said to OBS to be for his benefit more than mine.
I don't think it's to assuage my guilt as much as what I wrote above, but what I bolded really resonated with me, sisoon. Since I can see my reasons are mixed (and probably incomplete-- I'm sure there's even deeper stuff), and given the circumstances... I can't in good conscience say telling him would be MORE for the OBH than for myself and/or (in my perception) my WH. It's most certainly for OBH-- at least if I think about what I'd want** in his shoes. But I can't say it's MORE for him, not clearly. So I think I might let this sit a while. Thanks so much.
*"First AP" from when we were dating was a complicated issue where he took advantage of her and I assign her zero blame. He needed to make major amends to her and I encouraged his communication with her recently. She also sees him as very much "Not a Good Guy"-- basically no one has seen WH's bad side like I have except for her. So for him to confront this recently was huge.
**I also wanted to acknowledge that I don't know what it's like as a BS to have divorced, moved on, possibly be remarried, etc. with whatever DDays probably existed for them to be at least ~10 years in the past (vs my 10 months). So maybe even I-- for whom truth is perhaps unusually paramount-- would not feel I needed to know if I were in the same position OBH is.