Driver can no longer be uninstalled from the Menu in SR8.0?

Correct, as of Ventura 13.3, you can not uninstall the SoftRAID driver. It is a protected system extension, it cannot be "corrupted/damaged" and cannot be uninstalled.
this is a very good thing, its why you no longer need to "reduce security" to install SoftRAID.

So how do you downgrade the Driver, for example, from 8.0 to 7.6.1?

@henry-in-florida 

You simply cannot downgrade the driver. MacOS determines which driver loads.

Ventura 13.3 loads 7.5.

Sonoma from 14.0 to 14.3.1 loads 7.6.

Sonoma 14.4 loads 8.0

etc etc. You cannot change this.

You can very easily downgrade the app, just by running 7.6.1. (which runs in all versions of Sonoma)

The biggest win is, remember, you can run SoftRAID will full security enabled, which users have wanted since these requirements started with the T2 chip. Remember how complex it was to install and get the SoftRAID driver to load with all the MacOS security requirements.

We spent 3 years trying to get this accomplished (changing the SoftRAID driver to be a protected system extension) and users already want to undo it!  ;-)