When writing notation it is far easier to write a tune in simple 4/4 time and tell the player to play with a swing feel than it is to write the same tune with a 12/8 time signature. The 12/8 time signature woud ensure that the music got played correctly, but it's a pain to write. 4/4 is much simpler to write but it needs the instruction to interpret it as 12/8.

This comparative simplicity plus instruction will have been the same centuries ago as it is now.

The dotted crotchet, or quarter note, followed by a quaver or eighth note gets close. It is simpler to write than 12/8 but does not actually sound right.

Today we write something like 'Swing' or 'Play with swing feel'. Does anyone know what the equivalent was centuries ago?