I was under the master tier sub for years when this first opened up but eventually unsubbed due to not using it. Now I've come back and literally none of the content I'd purchased before is listed anymore.

Is this normal?

No.

Because of the way DDB has you sign in to the site, it's possible you had two different accounts, linked to different services (apple, google, twitch), and you logged in to the wrong one. (The account you're posting with dates to only 2022, which, if you've been on the site since it first opened up, suggests this is the problem.)

Also, you might as well check your product licenses and see if anything's listed there.

I'm signing in with Twitch because I remembered that's what I used at the time and all of my content is definitely gone. I'll have to search for the old emails about everything I bought.

What's the best avenue through which to get someone to look into this for me?

Honestly it's so long ago, I thought my username was harrisben if I could have a username or my email ben.harrisben@gmail.com.