I just saw an old blog post from Proton about joining forces with Standard Notes. It’s from 2 years ago and it said:

In the coming months, we hope to find ways to make Standard Notes more easily accessible to the Proton community>

I never even heard of it until someone alerted me to it, as i was looking for a new note taking app. I landed on Obsidian, but i thought it might be useful to have my notes incorporated in my proton account. So, i’m suspecting it is basically dead?

Proton keeps adding new stuff, shelving older plans indefinitely, that’s my impression anyway.