You can unlock transmog ensembles for your alts in Dragonflight even if you can’t use that armor type, but this does seem to be currently limited to Dragonflight ensembles.
MrGM discovered that you can unlock transmog ensembles for your alts on a single character. Let’s say you main a Priest (Cloth Wearer), but would like a Dragonflight ensemble for your Death Knight (Plate Wearer). With the new change, your Priest can buy and learn the Death Knight ensemble (set appearance) and your Death Knight can wear it.
However, the change does not appear to retroactively apply to older ensembles.