Udemy now expires all free coupons older than 30 days. This is to prevent "coupon abuse", though they don't say what that actually is.
It's in Udemy's interest that people pay as much as possible for every course they take. Some instructors use free coupons for a class intended to be free, to attract customers who might later pay for a course. Yes, you could make a course free from the get-go, but experience shows that those who take such a course tend to discount it, to give it exceptionally poor evaluations (ratings). That's evidently just part of user mentality.
I have two such courses working via free coupons. Even if I make a new coupon when the old one expires, that new one will not be in circulation, that is, people won't see it.
Both from student and teacher points of view, I recommend Skillshare over Udemy.
"The worst form of inequality is to try to make unequal things equal." -- Aristotle
"Always do right--this will gratify some and astonish the rest."Mark Twain
"A designer knows he has achieved perfection not when there is nothing left to add, but when there is nothing left to take away." Antoine de Saint-Exup'ery
"Not everything that can be counted counts, and not everything that counts can be counted." Albert Einstein
"Make everything as simple as possible, but not simpler." Albert Einstein