Operating system: Fedora 27 (4.15.8-300.fc27.x86_64). When I updated the system on 3/15, plasma was updated to 5.12.2.1. After a restart, since the kernel was updated during that batch of updates, my mouse was no longer a left-handed mouse, and there was no way to make it left-handed. The "Mouse" section of System Settings > Input Devices had disappeared.
-> The "Mouse" section of System Settings > Input Devices had disappeared. Same with me.
This is fixed in plasma-desktop 5.12.3-2 I think 5.12.3 will reach updates-testing soon.
Thank-you. It has reached updates-testing in Fedora 27. As soon as I installed it, my mouse automatically switched back to being left-handed and the Mouse section in System Settings > Input Devices has appeared again.