For alias addresses they are filtering that because the data thiefs and middlemen know a small portion of people do this. They cut off the plus or dot part when

For alias addresses they are filtering that because the data thiefs and middlemen know a small portion of people do this. They cut off the plus or dot part when your domain has a well known MX/MTA connected to it. That still leaves custom mailservers and non-alias (as in aliases via plus or dot, not actual mailserver aliases) which does give an indication as to who is sharing your data.