Follow-up:
I posted a poll on Threads about #alttext (which likely nobody will see) and I replied to my own message with a photo with an alt text description.
Once the photo is published I cannot read the alt text I had written. Am I doing something wrong? The post is here: https://www.threads.net/@elenarossini/post/DAS8jb-Ntl-?xmt=AQGzU6RVWnVjBvoDGP6ft8CGHZwncy-1-vRAQxtB7ldFRA
And sorry for linking to a Meta website, but for the record I only use Threads to post messages about the #Fediverse π€