Do Not Disturb hides the notification
It is in Quick Settings, at the bottom right of the screen. With it on, a reminder still reopens its page, but the notification waits silently in the tray.
A Chromebook has no separate reminders app any more. For a meeting or a chore, Google Tasks and Calendar do the job. For a web page you need to come back to, Latr reopens the page itself at the time you chose.
Built in
For a call, a bill or a deadline, ChromeOS already has what you need, and you do not need to install anything.
Both notify you in the ChromeOS tray. What neither does is bring back the web page the reminder was about: you get a line of text, and still have to go and find the page.
For a web page
One click on the listing, no account. Then pin the icon from the puzzle-piece menu so it is always one click away.
The page is saved at your default time and the tab can close. For a particular time, click the icon and type it the way you would say it: “tomorrow 9am”, “Friday afternoon”, “in 2 hours”.
At that time the page opens again, scrolled to where you stopped reading, and a notification appears in the tray. Snooze it from there if now is not the moment.
ChromeOS
It is in Quick Settings, at the bottom right of the screen. With it on, a reminder still reopens its page, but the notification waits silently in the tray.
On a Chromebook the browser runs for as long as you are signed in, so reminders are on time all day. With the lid closed nothing runs; on waking, Latr delivers what came due and says it was late.
Questions
Not a separate one. Google Assistant reminders were moved into Google Tasks, so on a Chromebook a reminder is a task with a date and time in Google Tasks, or an event in Google Calendar. Both notify you through the ChromeOS notification tray, and both are the right tool for a reminder that is not about a web page.
Install Latr from the Chrome Web Store, open the page, and press Alt+Shift+Y - or click the Latr icon and type a time such as “tomorrow 9am”. You can close the tab. At that time the page reopens on its own, scrolled to where you were, and a notification appears in the tray.
Not while the lid is closed: nothing on the machine runs then. When you open it again, Latr delivers anything that came due while it was asleep, and says it was late, so nothing is lost. While you are signed in Chrome is always running on a Chromebook, so reminders are on time for the whole working day.
Check Do Not Disturb, which lives in Quick Settings at the bottom right of the screen. With it on, ChromeOS keeps notifications in the tray without showing them. The page still reopens on time either way.
If your administrator allows extensions from the Chrome Web Store, yes. Managed Chromebooks often limit which extensions can be installed, and that is a policy only the administrator can change.
Free, with no account. Reminders are kept on the Chromebook, and if Chrome sync is on they travel to your other computers with the browser’s own sync.
Free, no account, and nothing leaves your device. About ten seconds to install, and one keystroke to use.
Also on Brave, Vivaldi, Arc and Opera - straight from the Chrome Web Store listing.