When were numeric contractions for ordinals first used, as in 1st, 2nd, 3rd, 6th instead of first, second, third, sixth?
According to Wikipedia, in Latin, ordinals were indicated by superscripts on Roman numerals.
Not all languages currently do this; for example German and most Eastern European languages do not. Most Romance languages do, along with a number of others, including Dutch and English.
In English, Wikipedia says these started out as superscripts: 1st, 2nd, 3rd, 4th, but during the 20th century they migrated to the baseline: 1st, 2nd, 3rd, 4th.
So the practice started during the Roman empire, and probably was continuously used since then in the Romance languages.
I don't know when it was adopted in English. Here is a pamphlet entitled:
Mr. PRYNNE's New-Year's-GIFT,
to the Rump-Parliament &c.
The 1ſt of January, 1648-9.
So it goes back a long way … I would suspect that you can find these contractions near the beginning of printed matter in English.