It was possible to use Html in the Select button message up to R1 2023. The current behavior is a regression in R1 2023 SP2.
Dojo example: https://dojo.telerik.com/uyuhocUL
No actual Html can be used in the button's message.
Html can be used in other messages of the Upload, e.g., dropFilesHere, so it should be possible to do it in the Select button message as well.