It’s not good to ever keep random accounts abandoned. It’s just good security and often privacy hygiene that most people don’t practice. I already have over 100 accounts total across the internet, and am planning to skim that number down drastically. It’s especially important security & privacy hygiene, because if you leave dozens of accounts abandoned and the services experience a data breach, the fact that your data is now out there is partially your own fault, and since we’re on the english-speaking side of the internet, I doubt we’re going to hear about a data breach for some random Japanese site.