Prior to W95/NT4 windows didn't even have a license key, scary warning text was the upper limit of enforcement. In some companies part of the IT departments job was to find unlicensed software and delete or pay for it.
Letting people pirate your software early on is a valid business strategy. Enterprise users pay, students/hobbyists find a simple crack. Once those students enter the workforce they decide the market.
I know that a few pieces of software distributed on floppy showed a warning if it detected it was previously installed (which wrote to some file on the disk, given it wasn't write protected). Which basically amounted to a sternly-written paragraph to say that they're using honor system to make sure you follow the rules.