Updating to Monterey on mid 2015 MacBook Pro I’ll loose what?

Big Sur uses APFS for the Time machine destination drive format, so that draws a line under those backups.

You can not use Big Sur or newer backups to restore Catalina 10.15 or older.

Catalina eliminated the use of 32-bit Applications.

You may be giving up some stability, because Monterey is so new.

Hi,

You'd better take backup before update to Monterey.

Also check applications sites if they support Monterey.

what MacOS are you running now?