Instead of focusing on MS-DOS, we could be talking about [DOS-Compatible]dos-compatible systems. This is probably similar to OP's 1st point, alas more generic.
I'm not sure if DoS or DDoS attacks are more frequent, but DDoS is definitely more powerful and annoying. I presume that every self-respecting attacker will either use a botnet or cooperation of his peers to attack, thus the [DDoS]ddos tag could work. [dos-attack]dos-attack also works fine, probably better.
I couldn't, unfortunately, find official stats to back-up my proposal, especially the second one, so if someone finds related references please DO comment!