MTN bike tyre sidewall wear question
From the knobs, that looks like a Continental X-King, but not the 'Protection' version as no checker pattern on the sidewall.

What you show in the pictures is quite normal to see on X-Kings (and some other tyres), and its quite safe to ride.  All that has happened is some of the very thin outer coating has worn off, but the structure of the tyre will be intact.

See review from Pro rider for an even more extreme example.