Discover linked email from steam account

Not unless it is recorded somewhere. This prevents people from being able to easily scan for email addresses to steal so that they can then steal the Steam accounts.

The next best thing is if you still have the phone number that Steam Guard was attached to. Recovery through that should be just as easy as with the email address.

If that fails you can contact support for help, but you'll need to have some info. For example the login name, numbers from a credit/debit card that's been used to make a purchase, or a retail Steam key (e.g. from a boxed game). They may also take info like the original phone number attached to Steam Guard, and the official support article above mentions "bank transfer" as one of the pieces of data they accept.