Custom ROMs are no longer as popular as they once were, but they still have their place in the Android world. Here's an example: the OnePlus One, a phone that came out in 2014, is now able to run stable Android 10 by way of LineageOS 17.1, which has just been released to the open-source project's download page.
When the OnePlus One debuted back in 2014, it was sold as an enthusiast phone with insane bang for the buck. Fast forward four years, and Android 8.1 Oreo in the form of LineageOS 15.1 has just been released for it. The OPPO Find 7a and Find 7s and Galaxy S5 Plus/LTE-A are getting 15.1 as well, and the Huawei Ascend Mate 2 4G has returned with LOS 14.1.