Why should they care though? Anyone with a catchall can create 7 billion normal looking addresses under their domain. It's not like it would prevent anything.
Also anyone with gmail address can also place dots almost anywhere into the local part, to create another unique address without using a + sign.
Because of spammers creating garbage accounts on your platform
Yeah, but why care about email address format, if it's not going to stop spammers anyway, and you're just risking losing legitimate customers if you mess up trying to mangle part of an address you should not be touching according to the recommendation in the standard?
The email spec considers these different emails. It's not a websites job to worry about how an individual host treats them. Gmail also treats . in the user section as useless but most hosts do not.
Not all email providers support the + notion so you'd have to run domain lookup on some hard coded list