While I agree with the first half of your statement (don't share SSH keys), I cannot agree with the second (don't put SSH keys in a password manager).
For my home use of 1Password, I absolutely want to keep backups of my SSH keys in 1Password. Because, in general, there's exactly 1 SSH key which can get into my cloud instances, and I've had enough laptops die suddenly that I'm not willing to risk getting locked out by not having a backup.
You could say "well, just have a second device with backup keys" but again for home use, why would I buy another laptop just for that? Or maybe just "well keep an offline backup of your keys". Sure. In 1Password. Where I keep pretty much all of my sensitive credentials and info.
> Using the 1Password SSH agent encourages people to have "one" SSH key across devices, which means that any leaks will disproportionately impact them.
Eh. IMO, people who are inclined to use 1 key across machines are going to do it, no matter the process. I doubt this feature is going to make that any worse. But I guess we shall see.