In case you're unfamiliar: it's a whole microgenre. However idk what to call it: afaik the more popular slowdown values are 400% and 800%, so ‘slowed down’ together with these can be used as keywords for searching.

E.g. https://youtube.com/watch?v=BFyeuLhjcPY

Slowing down music that was already downtempo gonna result in that ultra-ambient: https://youtube.com/watch?v=b3i06yw4A4E

‘PaulStretch’ software is often used for this, and its algo gives that ethereal quality.