I did not know all SM artists are guarnteed to get a solo
They’re not guaranteed to get a solo and some of the rappers have been put in duos but..typically all mains and leads plus popular members, and looking at EXO (just because I know it better than SNSD or SuJu), they’re going pretty deep in the well for solos. Xiumin is neither main nor lead vocalist nor one of the more popular members but he too recently had a solo. Knowing SM’s weakness in rap, those are usually the members who don’t get a solo or get their last while working on their vocals, I think. However, that will shift with NCT, where the rappers are strong enough to support solos much earlier (and among the most popular members).
Shinee - every member had a solo
EXO - every member has a solo except the rappers Chanyeol (likely to get one) and Sehun (unlikely to get one on his own IMO) (Chanyeol and Sehun already in subunit duo EXO-SC)
RV - every member has a solo except the rappers Irene and Yeri (Irene has sub unit with Seulgi) (Yeri might get one, unsure on Irene)
NCT - solos will likely start this year with Taeil (main vocalist) and one of these: Taeyong (main rapper, popular), Mark (main rapper, popular), Jaehyun (main vocalist, popular), maybe Doyoung (main vocalist, popular in Korea and Japan). I think everyone in 127 will get a solo (Johnny will be one of the last), not sure on Dream (SM’s track record with Chinese artists isn’t great) and pretty sure the WayV rappers wouldn’t get anything. Well maybe YangYang, who they also keep positioning as a singer lately.
SNSD - I think they all did? A sone can correct me here
SuJu- I don’t think they all did but they have lots of sub units and stuff so I don’t really know
TVXQ - both have solos
I don’t know about HOT or Shinhwa and f(x) didn’t last long enough..