None of what you lament has been lost. OTR still exists and is used by some folks. There are still OTR supporting clients that also support the current propriet

None of what you lament has been lost. OTR still exists and is used by some folks. There are still OTR supporting clients that also support the current proprietary messaging systems like Facebook. The only thing that was lost is that most people moved away from using Free Software for communication and most new people were born into and grew up within proprietary communication ecosystems. Please note that OTRv3 uses significantly outdated encryption. There is OTRv4 but it isn't finished yet. https://bugs.otr.im/otrv4/otrv4 https://bugs.otr.im/otrv4/otrv4 https://github.com/otrv4/otrv4/ https://github.com/otrv4/otrv4/