There has been an unannounced change that I think would annoy the community.

Gmail’s Mark as Unread as been moved out of the 3-dot top menu:

It’s not there when you check off a message in your inbox:

It still is available when you are reading a message, under the 3-dots menu on the right, but this is not helpful from your inbox!:

Here it is!

You can also right click on a message from your inbox: