This is normal but the message was disabled. What I find are folk will use a registry cleaner or tune up software and it turns off the setting that hides that message.

As this is the recovery drive, it's not for you to use.

Widely discussed so I won't duplicate all that has been written. Ignore it or turn off the low space warning.

BEWARE THOSE THAT WANT TO HELP YOU FREE UP SPACE ON D:. That could break the restore feature.