Shadowlands feature, should be doable using API.
C_LegendaryCrafting.GetRuneforgePowersByClassAndSpec accepts class, so just pass current player class here and filter available Legendaries.
C_LegendaryCrafting.GetRuneforgePowerInfo gives info about Legendary by its ID from the first function.

Some examples:
/script table.foreach(C_LegendaryCrafting.GetRuneforgePowersByClassAndSpec(3, 253, 0), print)
Prints all (available and unavailable) Legendaries for Hunter Beast Mastery.

/script table.foreach(C_LegendaryCrafting.GetRuneforgePowerInfo(70), print)
Prints info about the first Legendary from that list. Could use iconFileID I guess?

added