Samsung has had a one-handed mode in TouchWiz / Samsung Experience since 2012. LG V20 also have one-handed mode feature. Apple introduced Reachability to iOS in 2014. Remember iPhone 6 was launched in 2014 with a 4.7-inch screen size and knew right away that the users will suffer from one-handed usability. Other OEMs like Xiaomi and Huawei have added the one-handed mode to MIUI and EMUI respectively.
However, the One-handed mode isn’t restricted to OEM skins on Android. LineageOS, the most famous Custom ROM, jumped on the bandwagon and introduced the Single Hand Mode to the Custom ROM. The thing to note here is that LineageOS’s Single Hand Mode looks similar to Huawei’s one-handed Mode in EMUI. It’s actually based on Huawei’s code. One of the Huawei’s employee made commits to AOSP but there were just too many conflicts, therefore, it was never merged to AOSP. LineageOS took the code and modified it to the perfection. Single Hand Mode is now available in LineageOS 14.1 in an entirely stable capacity and without resulting in any kind of conflicts.
So, if you are using LineageOS 14.1 Custom ROM then you can use your device with one hand, thanks to Single Hand Mode.