Microsoft Office is locked unless I start a new subscription

LibreOffice. Intended to be compatible with Microsoft Office. Free. The Apple iWork Pages, Numbers, and Keynote apps can read and write many common Office documents. Also free. An Office 2019 purchase. Not free. An Office 365 subscription. Not free. You were running an ancient (2011 or earlier) version of Office, with known bugs and all, and the older Office version was built with older and Catalina-deprecated 32-bit APIs. Which means Office 2016 (no longer for sale), Office 2019, or Office 365. Or an alternative app. Or migrating away from the Office document formats and tools entirely. And here we are.

Load LibreOffice, or load iWork Keynote, or load the Office 365 subscription, or load the Office 2019 kit, or spend way more time and effort on this with rolling in your most recent pre-upgrade backup.

If you even have a path to downgrade available to you. Time Machine backups can be re-used after an upgrade, and the older macOS files can get deleted, for instance.

If you do have a pre-upgrade backup, you can roll that in, though that’ll discard everything that’s happened since the upgrade.

You could also roll that backup in as a guest in a virtual machine, though that's a technically more complex solution, and the better-known virtual machines available for macOS are commercial products; they’ll cost you.

LibreOffice, Keynote, pay Microsoft. You stayed on an antique version of Office well past its end of support, and the bill for those savings just came due.

You might find https://www.libreoffice.org (which is free) will be able to open your Office files.

Uninstalling the upgrade is not easy unless you have a full backup (like a Time Machine backup) in place.

You will probably find you can use the 'Impress' part of https://www.libreoffice.org to open your PowerPoint presentation for your meeting.

One final thought to add to the useful comments above. If you need some time to decide which way to go, you can use Microsoft's office free for a trial period and get your files finished and printed. Then you need to decide what to do.