We don't read "kisvei kodesh" on Shabbos?

Question

The Sefer HaMatamim (Shabbos 1) explains the reason we say Migdol instead of Magdil in bentching is because

מגדיל היא בכתובים בתהלים ואין קורין בכתבי הקודש בשבת - Magdil is part of Kesuvim in Tehillim, and we don't read 'kisvei kodesh' on shabbos.

Although I am aware there are many who don't say Tehillim on Shabbos because it is beseeching Hashem, which we don't do on Shabbos, but the Sefer HaMatamim seems to be suggesting another issue. What is the issue with reading 'kisvei kodesh' on Shabbos?

Answer 1

The grain of this prohibition and its exclusions, to which SH is alludes, is from the Mishna in Shabbat and Talmud (116b). Although the mere recitation of a verse or word, especially of liturgical nature, from Writings is not of Talmudic origin, as far as I know.

Answer 2

As DoubleAA referenced, this starts in the Talmud with Shabbos 116b, then codified by Rambam -- his Shabbos 23:19 reads:

אף לקרות בכתובים בשבת בשעת בית המדרש אסור גזירה משום ביטול בית המדרש שלא יהיה כל אחד יושב בביתו וקורא וימנע מבית המדרש

Even to read the Writings on Shabbos at the time of study assembly is prohibited, as the study assembly could become empty -- we wouldn't want everyone sitting home and reading, rather than attending the study hall.

Basically when study relied on communal sessions (recall that a great deal of the law in the Talmud was still based on oral teachings, including so much of practical halacha), we didn't want people missing those. Rashi on the Gemara noted that "at the time of study hall" meant before the main meal of the day; once people feasted, they might have a couple drinks to go with it.

As your local Orthodox rabbi how all of this applies today...