If you have a basic group chat that already has Pinned Messages, then you upgrade the basic group chat to a supergroup, it won't be possible to unpin them.
Workaround
Deleting the old messages from the chat history will unpin them.
Steps to reproduce
Note: You can change History setting to Visibile instead of Hidden, or something else, to force a supergroup upgrade.
Create a New Group
Send some individual messages to the basic group chat
Pin all messages in the group
Upgrade the group to a supergroup
Long press on a previously pinned message
Current result
Unpin button is not shown in the context menu, therefore the message can't be unpinned.
Expected result
Unpin button is shown in the context menu, the message can be unpinned.
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