Read this Travel Ban Explainer by Immigration Monitor (a credible guide to global immigration).
This lists countries with full and partial travel bans that affect various kinds of immigration and non-immigration visas – and the rationale for restricting travel and visas from these countries. Some exceptions make be available for some people with these reasons: (1) their residency/citizenship is in the national interest, (2) they would otherwise suffer undue hardship, (3) the pose no security risk.
Immigration Insight 