@Streamer317 is suggesting you log into your Roku account at https://my.roku.com and unlink the device from your account there. There is no 3-dot control there, just an "unlink" link.
Roku Community Streaming Expert I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well. If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X
Re: How can a remove an ancient Roku HD device from the app?
I understand what you are saying. That particular device doesn’t show up in “my linked devices” when I log in to my account on https://my.roku.com but It does show in “devices” on the Roku iPhone app. I think it’s a matter of just ignoring it when I see it on the iPhone app. Thanks for you suggestions.