Bug Minecraft.

What did you expect? To not be able to start your chat message with a "t"? To not be able to type the "/" when opening a command line?

Not a bug.

Then why when pressing a slash 2 slashes are written. This was not the case with older versions?

you hit the forward slash ... many times

That's not what you reported, you reported pressing it many times.
So which is it? Pressing it once (and not holding it down), or pressing it many times?

If you press and close the chat with / and T many times, the bug will work.

You cannot close the chat/command bar with those buttons, only the escape button.

The same must be done with T. Only you need to press the T button.

Cannot reproduce, you probably just pressed the button twice in a row on accident.

No. Here is a video that reproduced the bug.

In your video, there are clearly frames before the second slash gets added, meaning your keyboard registered 2 keypresses.
Again, cannot reproduce; this is looking like a technical issue.

Thank you for your report!
However, this issue is Invalid.

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