Imperfect but good and free
I tried half a dozen slitherlink games, and all of them were useless—either there’s no generator for new games beyond the couple dozen it comes with, or you have to wait for “stamina” to play for new games, or you’re bombarded with ads, or there are glitches that make it unplayable, or the interface is unusable. Then I found this one. No unnecessary “gamification”, no timers, just pick it up and play a custom slitherlink whenever you want. The interface doesn’t scale well as you play larger puzzles and it has to shrink everything down (it starts to shrink beyond 10x8 on an iPhone 11, but the cutoff probably depends on your device, and how far it can shrink before it becomes too annoying probably depends on your eye and fingers). A few of the color schemes (which it randomly selects each time) are pretty low-contrast. But those are the only problems I’ve found, and they don’t negate the fact that this stands head and shoulders above every other slitherlink app I’ve tried in that you can actually have fun playing it.