Earlier quoted context omitted.
The next time you apply for anything that depends on your credit history, the collections history will show up on the credit report (assuming you’re in a country that has those - far more than just the US) and either cause your application to be refused or to be approved with bad terms. This will stay on the credit report for years. It’s probably not worth anyone’s time to go after you more aggressively than that mer…
It never showed up on my credit report and I've not had any problem with any sort of application in the subsequent years.
Maybe you got a rare debt collection agency that doesn’t bother, or one which decides your debt was too small to be worth their bother for that, or maybe your debt collector is actually the accounts receivable department of the gym (which probably isn’t set up to submit credit reports) rather than an external collections agency (which usually would be set up for that).