I think Keybase does a decent job here. You can invite someone to a team by email, and they need to install the app, pick a username, and follow the join team

I think Keybase does a decent job here. You can invite someone to a team by email, and they need to install the app, pick a username, and follow the join team instructions. It's probably technically not as easy for a one off situation as onetimesecret.com, but the app download, setup and chat are pretty familiar UI models for many people. In the long run, it gives you multiple ways of sharing that secret. Encrypt it and stick in the wiki, use an encrypted git repo, exploding messages via team chat, etc. That being said, a real password manager is the proper tool here. I personally like LastPass, and it does require registration but has "1-to-1" sharing in the free version.