Noticing that the title of this isn't original itself (or is it?) There's a similar post entitled "Combinatorial Creativity and the Myth of Originality" by Maria Popova at https://www.smithsonianmag.com/innovation/combinatorial-creativity-and-the-myth-of-originality-114843098/

Perhaps the William Inge quote is incredibly apropos here: https://hypothes.is/a/Fvkz-i8rEe2hJYM4oINfpw

reply to u/stjeromeslibido at https://www.reddit.com/r/Zettelkasten/comments/10qw4l5/are_there_symbols_for_supported_by_or/

In addition to the other excellent suggestions, I don't think you'll find anything specific that that was used historically for these, but there are certainly lots of old annotation symbols you might be able to co-opt for your personal use.

Evina Steinova has a great free cheat sheet list of annotation symbols: The Most Common Annotation Symbols in Early Medieval Western Manuscripts (a cheat sheet).

More of this rabbit hole:

• Steinová, Evina. Notam Superponere Studui: The Use of Annotation Symbols in the Early Middle Ages. Brepols, 2019.
• Cappelli, Adriano. The Elements of Abbreviation in Medieval Latin Paleography. University of Kansas Libr., 1984.
• Coulson, Frank, and Robert Babcock. The Oxford Handbook of Latin Palaeography. Oxford University Press, 2020.
• Lindsay, W. M. Notae Latinae. Cambridge University Press, 2013. https://archive.org/download/notaelatinaeacco00lindrich/notaelatinaeacco00lindrich.pdf.
• Bains, Doris. A Supplement to Notae Latinae (Abbreviations in Latin Mss. of 850 to 1050 A.D.). Cambridge [England] University Press, 1936. http://archive.org/details/supplementtonota0000bain.

(Nota bene: most of my brief research here only extends to Western traditions, primarily in Latin and Greek. Obviously other languages and eras will have potential ideas as well.)

Tironian shorthand may have something you could repurpose as well: https://en.wikipedia.org/wiki/Tironian_notes

Some may find the auxiliary signs of the Universal Decimal Classification useful for some of these sorts of notations for conjoining ideas.

Given the past history of these sorts of symbols and their uses, perhaps it might be useful for us all to aggregate a list of common ones we all use as a means of re-standardizing some of them in modern contexts? Which ones does everyone use?

Here are some I commonly use:

Often for quotations, citations, and provenance of ideas, I'll use Maria Popova and Tina Roth Eisenberg's Curator's Code:

• ᔥ for "via" to denote a direct quotation/source— something found elsewhere and written with little or no modification or elaboration (reformulation notes)
• ↬ for "hat tip" to stand for indirect discovery — something for which you got the idea at a source, but modified or elaborated on significantly (inspiration by a source, but which needn't be cited)

Occasionally I'll use a few nanoformats, from the microblogging space, particularly

• L: to indicate location

For mathematical proofs, in addition to their usual meanings, I'll use two symbols to separate biconditionals (necessary/sufficient conditions)

• (⇒) as a heading for the "if" portion of the proof
• (⇐) for the "only if" portion

Some historians may write 19c to indicate 19th Century, often I'll abbreviate using Roman numerals instead, so "XIX".

Occasionally, I'll also throw drolleries or other symbols into my margins to indicate idiosyncratic things that may only mean something specifically to me. This follows in the medieval traditions of the ars memoria, some of which are suggested in Cornwell, Hilarie, and James Cornwell. Saints, Signs, and Symbols: The Symbolic Language of Christian Art 3rd Edition. Church Publishing, Inc., 2009. The modern day equivalent of this might be the use of emoji with slang meanings or 1337 (leet) speak.

Others have noticed!