https://www.mangaupdates.com/series.html?category=Star-Crossed+Lovers&orderby=rating
Should be some. It'll include unable to be together for other reasons too though
In BL, there're many. Have you read these?
http://www.mangago.zone/read-manga/unknown_letters/
http://www.mangago.zone/read-manga/anata_no_tame_nara_doko_made_mo/
http://www.mangago.zone/read-manga/500_nen_no_itonami/
http://www.mangago.zone/read-manga/omerta_kochou_no_yume_no_kate/
http://www.mangago.zone/read-manga/almost_paradise/
http://www.mangago.zone/read-manga/not_equal/
try to use baka-updates
https://www.mangaupdates.com/series.html?act=genresearch
you can search for genre + category
in this link I wrote forbidden love you only have to mark your genre (yaoi, drama...)
ah you can also use more category words like Social Gap
you only have to spell the word in the right way (it has Social Gap but it hasn't social gap)
here you can search for your category word https://www.mangaupdates.com/categories.html
oh it is difficult yet i find the concept very interesting so i collect them as my precious. you can visit my personal collection of star-crossed lovers http://www.mangago.zone/home/mangalist/43923/
i also separated my straight mangas in the same collection, if you want to check them out http://www.mangago.zone/home/mangalist/81751/
related list of mine is age gap (features social standing gap) http://www.mangago.zone/home/mangalist/93734/ and cinderella man (financial / breeding gap) http://www.mangago.zone/home/mangalist/68437/ so YES. if it isn't obvious already, i'm so into it hahaha.
Have you tried "Yuuutsu Na Asa" by Hidaka Shoko? By far One of the Most Beautiful Angsty Love Story I have ever come across. The characters are just so pitiful yet so very endearing. It's one of those mangas that Ripped my heart to pieces but I still keep going back to it like a freaking Masochist! ╮( ̄▽ ̄)╭
http://www.mangago.zone/read-manga/yuuutsu_na_asa/
whenever I read YNA I always feel like yelling "take my heart and just effing rip it to pieces you, you bad boys!" I can't emphasize enough how good it is. Pity other people dismiss it because it's "historical". It should be at yaoi's top ten at least.
Nah, me? I'm just a drama queen. YNA is just that good I must get back over and over.
Really? People do that? I never knew. If that's the case then they don't know what Pure Gold they are missing! Sure, the story is Heart wrenching to the point your heart starts to Bleed but I personally don't think any other Mangaka besides Hidaka Shoko can pull off a Masterpiece of this caliber in the Historical setting. It's easy to build a Great plot but it's very difficult to keep the consistency in the long run.
I never knew how hard it was to find a manga with a plot of two people forbidden to be together due to political/familial/social status/ etc etc differences...