Code of Conduct
What article on docs.github.com is affected?
The page I found the issue on is:
https://docs.github.com/en/subscriptions-and-notifications/how-tos/managing-subscriptions-for-activity-on-github/viewing-your-subscriptions#reviewing-all-of-your-subscriptions
The pertinent step in this article comes from a reusable, so it may also affect other pages. The reusable is:
https://github.com/github/docs/blob/main/data/reusables/notifications/access_notifications.md
What part(s) of the article would you like to see updated?
In the "Reviewing all of your subscriptions" section of the webpage, Step 2 requires the user to be on the Notifications page (https://github.com/notifications). Thus, Step 1 needs to lead the user to that page. Step 1 contains the aforementioned reusable, which provides the following instructions:

(screen capture included above due to relevant icons/images)
The problem with these instructions is that it is possible for a user's GitHub webpage to not show the inbox icon. The user cannot click an icon that is not present, so for users in this situation alternate instructions should be provided to lead them to the Notifications page successfully.
Additional information
For context about what circumstances result in the inbox icon not showing in the GitHub website's UI, here is the explanation as described by the GitHub Support member who helped me troubleshoot my lack of an icon:
Thanks for the additional information. I was able to reproduce the behavior on my side.
The Notifications button is removed from the header when On GitHub/web notifications are disabled for the account. This also explains why the button appears briefly during page load and then disappears once your notification settings are applied.
You can enable it from Settings > Notifications by enabling On GitHub notifications for the notification types you want to receive. Once enabled, the Notifications button should remain available in the header.
My earlier statement that the button should always be present was incorrect - apologies for the confusion, and thanks for helping us narrow this down.
Code of Conduct
What article on docs.github.com is affected?
The page I found the issue on is:
https://docs.github.com/en/subscriptions-and-notifications/how-tos/managing-subscriptions-for-activity-on-github/viewing-your-subscriptions#reviewing-all-of-your-subscriptions
The pertinent step in this article comes from a reusable, so it may also affect other pages. The reusable is:
https://github.com/github/docs/blob/main/data/reusables/notifications/access_notifications.md
What part(s) of the article would you like to see updated?
In the "Reviewing all of your subscriptions" section of the webpage, Step 2 requires the user to be on the Notifications page (https://github.com/notifications). Thus, Step 1 needs to lead the user to that page. Step 1 contains the aforementioned reusable, which provides the following instructions:

(screen capture included above due to relevant icons/images)
The problem with these instructions is that it is possible for a user's GitHub webpage to not show the inbox icon. The user cannot click an icon that is not present, so for users in this situation alternate instructions should be provided to lead them to the Notifications page successfully.
Additional information
For context about what circumstances result in the inbox icon not showing in the GitHub website's UI, here is the explanation as described by the GitHub Support member who helped me troubleshoot my lack of an icon: