Any reason why I couldn’t tap a thread in to an alloy steerer on a pair of forks or where they all steel steerer’s when 1″ 1/8th threaded headsets were around?

IIRC Alloy steerer tended to either:

1) have a thicker wall reducing the internal diameter meaning a quill stem will not fit

or

2) if the forks are more modern there may not be the material on the steerer to support a thread being cut.

Clearly there are exception to the above but they tend to be more period forks i.e. alloy steer tubes designed with quill stems in mind.

Steel steerers shouldn’t present a problem.

So does that mean I cannot use an alloy steerer in a threaded headset?

you can use an alu steerer but I would heavily suggest one that was designed for the job from the outset. threading one that isnt designed for it could lead to failure