No, Microsoft has not made older Surface Bluetooth mice incompatible with newer Surface Pro models.
A 2019 Surface Bluetooth mouse should work with a 2026 Surface Pro.
Make sure the mouse has working/fresh batteries.
On the mouse, press and hold the pairing/Bluetooth button for about 3–5 seconds until the LED flashes.
On the Surface Pro:
Settings → Bluetooth & devices → Add device → Bluetooth
Select the mouse when it appears and connect it.
If it doesn't appear:
Turn Bluetooth off/on on the Surface.
Restart the Surface.
Disconnect the mouse from the old computer.
Remove any previous mouse entry and pair again.
Make sure the mouse's LED is actually flashing.
You do not need to buy a new Microsoft mouse just because you have a 2026 Surface Pro. The old Bluetooth mouse should work unless there is a problem with the mouse, batteries, or Bluetooth pairing.