Two games have been removed from My account

So I've looked through and it only goes back as far as September 20 2017. What do I do from here?

If you're 100% sure you're using the account that purchased them, contact support as mentioned above.

Achievements wouldn't prove ownership of a game. It's possible to play a game even though someone else bought it. You'd want to check through your transactions to see if it shows the purchases for those games on your account.

so I went and checked my refunded/return games, and neither assassin‘s Creed unity or terraria appeared. so I do not believe that they were refunded/returned but I could be mistaken. If you need any proof that I owned the games back in those times, I do have screenshots and achievements proving that I did have them.

No worries, in cases like these you should contact Xbox support so that they can verify your account, because here in the community, as we are independent consultants, we do not have access to this type of information.

If you have a secondary account, check that the games are not on it.

Go to: https://support.xbox.com/en-US/contact-us/

See the bottom left part of the page and make sure that the page has the correct region that you are in (ex: United States - English for USA)

If not, click on it then select the correct region that you are in.

Check and confirm that you are within the available time of support under Contact times.

Click Contact us

Select the drop-down menu options that will match your concern.

On the page, see the provided support articles that may help you with your concern. Click See contact options if further assistance is needed.

On the next page, select either Chat online or Request a call option then follow or enter the needed details.