> if not is there really a point to include everything possible in unicode ?

Needing to load three fonts to show a single document that mixes vastly different character sets is still infinitely better than not being able to have those different characters in the same .txt or .md file at all

> how many space is remaining for new alphabet and smileys ?

Unicode can encode about 1100k code points, and about 800k of those are currently unassigned and available for future scripts or characters

As an example of having not-exactly-a-character as Unicode "characters", it is rather rare that musical symbols are embedded in running texts (which is a primary litmus test for encoding), but musical symbols are typically rendered with existing font technology so there are needs for standardized "character" codes, as SMuFL [1] does. In fact Unicode 18 will get tons of musical symbols that have been in SMuFL for a long time but not yet in Unicode [2].

[1] https://www.smufl.org/

[2] https://www.unicode.org/L2/L2025/25017-miscellaneous-musical...

> 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

The Noto fonts have great coverage: https://notofonts.github.io/overview/