A new comments (preview) feature has been added recently in the Modern App Designer
To see it in action, open an existing app or create a new app using the Modern App Designer.
We can see the Comments (preview) options in the toolbar.
Comments allow us to add notes, discuss with colleagues using mention etc.
We can add multiple comments and it shows the count.
The comments can be edited, deleted, and resolved.
This is how the resolved comment look like – disabled
We can add comments to the page of type – view, form, dashboard, and custom page.
We can add comment also within the Navigation bar.
Currently, mention is not listing any suggestions –
Check here for more such features added – https://powerapps.microsoft.com/en-au/blog/round-up-of-the-ignite-announcements-for-power-apps/
Hope it helps..