Thank you!

1. If you look at @justyy's witness ranking table, only 4 out of the top 50 witnesses do not publish a price feed. While it's technically optional, in that your witness will still produce blocks without publishing it, the price feed is ultimately required for Steem to function correctly. So, if you want witness votes, I think it's basically essential :)

2. Yes, that specific stop key "STM1111111111111111111111111111111114T1Anm" is still the correct way to deactivate your witness.

I think the original idea of this particular stop key is that it would parse as a valid key in that its checksum is valid, but it's obvious to look at it that it could not have been generated randomly - therefore it's impossible for anyone to have the corresponding private key.

However, it doesn't actually seem to parse as a valid key! Various tools have a special case for it.