"Twitter/X still exists" means it is possible for people to make posts on and reference previous posts made on Twitter/X, no matter what it is named or who it is owned by at that point. If this is not the case, this market will resolve N/A.
"Elon Musk is in control of Twitter/X" means that it is still possible for Musk to make arbitrary changes (eg starting Twitter Blue, changing the site's name, adding view/bookmark counts, etc) to the website with almost no obstacles. If Musk retains some power over Twitter/X, but others veto some of his decisions in practice, that counts as a NO resolution.
Situations that would result in this market resolving YES:
Musk is the outright CEO of Twitter/X
The current situation, where Musk is not officially CEO but remains the owner
Musk sells the company to a friend, or IPOs while keeping majority share, such that in practice policy decisions remain his
Situations that would result in this market resolving NO:
Musk sells or IPOs Twitter/X such that he loses his absolute control
Twitter/X is repossessed by creditors, who subsequently remove Musk
Musk passes away before EOY 2026
Since resolution criteria are partially subjective, I will not trade in this market. If the resolution is non-obvious, I will discuss in comments with traders before resolving.