Even in 1990, NOSS allowed any user to browse a hierarchical directory (showing contact info, job titles, manager/reporting relationships etc), email or instant message anyone, and look at their calendar to see what they were doing. It was 10 years before you could do all those things using Microsoft Windows and Office. In recent years, Big Blue’s email environment has seemingly been less happy.
When Microsoft Exchange first came out, email was handled with the Exchange client and calendaring was from Schedule+, which had been updated to support Exchange (and lives on in some backward Microsoft lingo, where people who start every sentence with “So,” ask you to send them an S+, meaning, invite them to a meeting). Outlook came along in 1996 and became the preferred and unified way to do email, calendars, address books etc.
Some organisations had a default policy when new mailboxes were created that their calendar was shared Read-only, so anyone in the company could see it. You could open someone’s calendar fully in Outlook or by viewing the scheduling tab in a meeting, where you will typically see if a list of people are available or not. Others might have it that only free/busy info is visible by default, and that is pretty sub-optimal.
With M365 in general, newly-created mailboxes have no calendar sharing set up, and the action is on the user to choose how to let co-workers see their info.
Be a nice person, and check to make sure your colleagues can view your calendar.
Ideally, share so that others will see the title and location of any appointment; useful when someone is trying to arrange a meeting, as within the schedule view they can figure out if you are likely to be able to make the proposed meeting time – if your diary is full of blocks marked busy or tentative, they’ll have no idea if you really are in a meeting or have just marked time to do something that you might be happy to move. Or had a colleague’s FYI notice of being on holiday obliterate the view of your calendar.
In the early days of Exchange/Outlook, if you had read access to someone’s calendar, you could open up appointments, see who else was attending a meeting, download any attachments and so on, unless the appointment was marked “Private” – though it’s somewhat possible to open Private appointments programmatically if you know what you are doing.
Nowadays, calendar sharing is more granular – in Outlook Web App, go into Calendar and you’ll see the Sharing and permissions option, which will let you choose specific people and give them ability to see various details, or you can change the default for the whole organisation.
In full-fat desktop Outlook, click on the Share Calendar option on the ribbon, and you’ll get a 1990s-style dialog box allowing you to set the default permissions or to invite particular team-mates to have higher level access should you want them to know where you are and with whom.
If you choose titles & locations, viewers can’t open your appointments to peer inside, so can’t see who else is attending or what the body of the meeting says, but they can at least see if you’re likely to need travel time between meetings. See here for more info on calendar sharing & delegate access.