The Syriac abjad has twenty-two letters and has had twenty-two letters throughout its documented history. It did not grow, and the things that look like growth are all of another kind: ligatures written by scribes, positional forms belonging to letters that already existed, marked signs made for Garshuni, and letters added by other languages that borrowed the script. Taw closes the list, and the list has stayed closed.
That is not the usual outcome. Alphabets in wide use ordinarily acquire letters, and neighbouring scripts in this atlas did: Arabic writes six letters beyond the inherited twenty-two, Mandaic added two ligature letters and later a borrowed one. Syriac chose extension by marking instead, again and again, over a very long period and across several languages.
One reason is what the script was for. Syriac is the language of a Bible and a liturgy, copied by communities for whom accurate transmission was the point, and a fixed alphabet is easier to transmit than a growing one. Another is that the additions were mostly made for other languages, by writers who had no standing to alter the Syriac alphabet and no wish to.
The result is that a Syriac letter can be described without qualifying which century is meant. Taw is the twenty-second letter, it carries four hundred, it takes a hard and a soft reading, and those statements are as true of the earliest inscriptions as of a liturgy printed this year.