The answer hasn’t dissappeared. You actually asked that same question 3 times. You’ll find the version with the answer is still there and visible at: https://answers.ros.org/question/307821/ros2-and-custom-mutexes/
I think there might be a bug in the askbot UI that only renders each title once on your profile since you asked two questions with the same title. You only see 2 of the three on your profile.
Please be careful not to ask duplicate questions as this too 2 or 3 people to both answer your question as well as to triage them as duplicates and remove or close them. https://wiki.ros.org/Support