How do I find my very first email address?

Talk to people who you would have emailed with back then. I'm sure one of them has your old email address floating around in their inbox. Your parents?

R.I.P?
I think there was an additional agreement that would be removed if you haven't connected to your Microsoft account for two years.

Microsoft account activity policy
https://support.microsoft.com/en-us/topic/microsoft-account-activity-policy-7c0a9fa7-0982-b7c6-fd72-df852b63699f

Microsoft account activity policy: Under the MSA, you must use your Microsoft account to keep it “active.” Microsoft reserves the right to close all “inactive” Microsoft accounts. Please see section (4)(a)(iv)(2) of the MSAfor the consequences of a closed Microsoft account. Except as provided below, you must sign in to your Microsoft account at least once in a two-year period to keep your account active. If you don’t sign in during this time, Microsoft will consider your account to be inactive.

Exceptions: The activities below are exceptions that will result in Microsoft extending your account’s active status, even if you have not otherwise signed in to your Microsoft account within a two-year period.

In any case, you should check if you have any other exclusions.

Do you have some very old backups? It might be there.

No phonenumber connected to your email?

Hasn't Law ran away with all the steam gifts and closed Playblink though?

If you have an account associated with an older online game/website and you remember a user name (that isn't an email), you might be able to try resetting the password. Sometimes when you ask to reset password it will tell you the email, but it might not (don't do this if you aren't willing to lose access to whatever site you try this on). Failing that the only other option is if you have an old computer that might still have login details (saved via web browser or the such).