Why y/a discontinued in May?

Actually April 20. After that it's read only which makes no sense.

Probably because the site has been infested by trolls. A fair amount of adult content, even flat-out porn, gets posted here, too, certain groups are regularly slammed (take a look at the incels and MGTOWs attacking women over in Gender Studies), and the Trumpkins have been a real waste of space for the past several years.

YA is no longer the interesting, informative site it used to be.

It should have been shut down eons ago.  I won't be sorry to see it go.