Update: Google officially announced in Dec 2023 that all UA data would be deleted on the 1st July 2024, and notifications have been pushed via GA4 to alert users about this. The update was made to the existing support article already linked in this answer.
After July 1, 2023, you'll be able to access your previously processed data in your Universal Analytics property for at least six months. We know your data is important to you, and we strongly encourage you to export your historical reports during this time.
https://support.google.com/analytics/answer/11583528
Google have said that you'll have at least six months to review it, and you can export reports to other formats (Google Sheets/Excel/etc) during that time as normal. Do be aware that reports built in Looker Studio utilise live data, so any reliant on UA data will become inoperative after the cut-off date.
Exactly when UA data will become unavailable, I've heard differing reports of 1st January 2024 to 1st July 2024.
I know is that my agency are currently working through our clients' data and making relevant backups. We're not going to be able to get all of the data, but we're getting what's relevant to our clients (mostly conversions or other KPI by channel and month).