As is shown in the ticket, the extension installed in the Collection Scope
, the extension will be installed for all team projects.
Azure Devops doesn't support installing extensions for a single project now.
Since you are using the custom web page(custom extension), I am afraid that there is no method could limit the extension to users.
For some extensions and paid extension, you can restrict users from using this extension by assigning extension to user or changing access level.
You could refer to this doc about users access extension features.
Here are the User Voices about this feature, many customers have the same requirement.
Allow Extensions to be Installed only in Projects
Extensions scoped at a project level
You could vote it and add your thoughts in comment.