Who is being considered a Dead Witness?
The list you can see above shows all Witnesses placed from 71 - 81 in the current Ranking. As you might recognize a lot of them are actually Pinked in, which means that their Witness Servers are currently not running.
Of course it might just be due to some maintenance or other reasons, that they had to go offline for some time. In order to get to know whether this might be the case you can have a look into the last column, which shows the Version of the Steem Blockchain they are running. Crossed out Witnesses showing a Version 14, 16 or 19 can thereby definitely be considered as inactive and might get called "Dead Witnesses".
Remove your votes from the Dead!
In total there are currently 20 inactive Witnesses in the Top 100 with 15 definitely to be considered as Dead Witnesses. So basically this means many Steemians are still voting for inactive Witness accounts or to see it from another angle, didn´t update their Witness votes at all during the last months.
Therefore please consider to take a look on your current votes, whether they include some of those Dead Witnesses and remove them respectively. Rather give your vote to someone, who is active, running on the latest Version and really engaged into the Steemit Community. So maybe you´d like to consider to vote for someone like me, who´s running Community Projects like 
Clean up your Witness Votes right now on https://steemit.com/~witnesses