> how do they handle changes in scripts, for example if new proto-cuneiform or seal script symbols are discovered

They get added in the next Unicode revision.

In Unicode you have "blocks" [0] that are often bigger than the number of characters in a script, language or function. There are usually also space for new blocks between unrelated blocks.

For example, in the case of cuneiform, it was introduced in Unicode 5.0, and there have been revisions in 7.0 and 8.0 [1]

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  0: https://en.wikipedia.org/wiki/Unicode_block
  1: https://en.wikipedia.org/wiki/Cuneiform_(Unicode_block)#History