TLDR;

• Check your Password Manager/Stored Browser Credentials
• If on Apple devices, check your Keychain
• If on Android or using/used Chrome, check your Google Password Manager (enabled if you chose to save passwords to your Google account)
• Search old email inboxes
• Search for your email in data breaches
• Search for old usernames you re-used across sites

I personally would also add searching your browser cookies, since some browsers will keep around old cookies for years if you don’t clear them.

Does deleting old accounts really do much? Once you do something online, you should assume it’s there even if you delete it.

Best you can do is adopt a more privacy conscious way to go about things and just let your old footprint go stale