The opposite of not opening is opening. Eight-letter synonyms for opening, depending on context, include clearing, unlacing, exposing, starting
Shutting
shutting would work
Shutting
Closing
The word is roundest.
Standing
stopping
Teenager
One is shutting.
Imprison is a word which has the opposite to liberate and has eight letters
Stopping
approach, increase
Opening
dismount is the opposite of mount and contains eight letters