Good question - I see 2 problems here:
1.) the concept of a instant-messaging/chat: other than email-messages (which are indiviual "objects", elements,.. "things") a chat message is just a part of an endless stream, like a very long (text) scroll.
Example: If someone missed 5 messages and scrolls the text down to the most recent message: did this person actually read all 5 messages or just the newest one?
I do not see a method how any software may distinguish between "message just scrolled across the screen" and "message was read while scrolling across the screen".
Perhaps face-recognition - or better: eye-movement-recognition, may work since the eyes will move left to right when reading.
2.) Webex does offer a "seen by" indicator but it´s far from perfect since it disappears as soon as a new message is posted = you can only see if the most recent message was seen by User X, Y or Z... but you can not see any more how far each space-member did read.
I think this posting from me explains better what I try to say:
https://community.cisco.com/t5/webex-meetings-and-webex/question-to-forum-collect-visual-issues-here/m-p/4311006/highlight/true#M3852
