No such thing as "indispensable". Very few companies and projects die to a singular departure, you're just not that special kiddo.
What's actually happened here is a selfish desire to know things without sharing them. If you learn something new, share it somewhere.
"People came to ask the Oracle questions only the Oracle could answer for the Oracle refused to share knowledge by any other means".
Confluence is pretty good if you use it properly. Weekly knowledge share sessions are also handy.
Learning a thing and sitting on it makes you an asshat, not an asset. Seems OP learned the hardway, but they still need to remove their ego from the equation.
You're not good at knowing things (basic human capability that), you're terrible at sharing them.