Point 1:
About submission policy
,
We can also set the Submission Policy which dictates whether we allow
users to contribute to the Term Set (commonly referred to folksonomy) or restrict it to > only metadata managers (Taxonomy).
Submission policy has nothing to do with Stakeholders.
Point 2:
How to organize Metadata in the SharePoint Term Store
Please note that though Term sets have Owner and Stakeholder fields as
well, they do not control security for the given term set. Once again,
security for term sets is controlled at the group (folder level).
Security has nothing to do with the field(Owner and Stakeholders).
Point 3:
Managed metadata roles
When you set up a term set, you can designate a group or a person as
an Owner, Contact, or Stakeholders for the term set. These labels do
not grant any specific permission to work with the term set. Instead,
they provide a useful way to track the business owners or stakeholders
for a term set.
Only this Information available for Stakeholders, that s/he able to track it. How? didn't mention.
Point 4:
Creating new Term Sets In SharePoint 2013
users assigned as Stakeholders will be notified when the Term Set
changes
This used to happen in SharePoint 2013 and earlier versions.
Point 5(Practical):
I created a term set, assign group managers and contributor at the group level, assign Owner and Stakeholder at Term Set Level, but nothing happens.
- Performed CRUD on Term Set using Owner and then using Contributor
- Changed Properties of Term Set using the owner
I got no email notification. Please! check Screenshots.


Point 6:
After this much operation, what major changes SharePoint platform talking about, I don't know.
This information is used to track people and groups in the
organization that should be notified before major changes are made to
the term set.
Solution 1:
If it's really required for you. Then you can create a Powershell script which will retrieve all term sets created or modified today and send an email notification to stakeholders. This Powershell script executes on a daily basis.
Conclusion:
- I think it had only worked in SharePoint 2013 and versions before it.
You can generate a Support ticket to Microsoft.
- No configuration of
SMTP
and Exchange
required in SharePoint
online.
- Just for Information,
Alert Me
operation will work in SharePoint
online when adding a new list or library into SharePoint.
Update: 17 Sep 2019
I talk to Microsoft support and in reply, they provide me a text which is mentioned in Point 3.
Instead, they provide a useful way to track the business owners or
stakeholders for a term set.
Therefore the user who wants to make changes in the term set, first s/he will discuss with owner and stakeholders. This is the process in SharePoint Online, That's all.
Again, in the SharePoint server, they will notify when changes are made in term set but not in SharePoint Online.