It's not perfect. It's not always pretty. But for a free app it's kind of unbelievable what it can do.
I've been using Key Mapper daily since around Christmas. Not just for Sideswipe — for emulators, for a couple of gacha games my girlfriend plays, even for scrolling Reddit with a controller when my wrist was sore.
The thing that impresses me most isn't a specific feature, it's that the dev clearly plays games. Little details, like being able to set a hold-down duration in milliseconds, or the way it handles trigger axes — those aren't things you add unless you've been frustrated by not having them yourself.
Where it stumbles: the accessibility permission flow is genuinely confusing the first time. I've had to walk three friends through it and every time I forget which sub-menu the setting hides in on their phone. Samsung and Xiaomi bury it in different places. That's not really Key Mapper's fault but it's the biggest thing keeping newcomers out.
Would I pay for it? Yeah, honestly. I actually did throw the dev a few bucks through the Play Store. If a free app is saving you from an own-goal every match, that's worth a coffee.