Well, I suppose that the actual "it' shows up in canon" is the one already mentioned - the Bonisagus' "cool spell research" book.
But yeah - the main issue I've had with that is that it doesn't show up any other place. And you'd think, with a new version of that book circulating every seven years, at least THOSE experimental spells would show up in more wizard's spell lists.
Now - from a meta standpoint, I can take a reasonable guess - it's an additional layer of complication that the writers didn't want to take for character creation - as it would require there be a "common core" of experimental spells (such as, say, from the aforementioned Bonisagus book) that folks learn from; and then the writers would have to determine which versions hold what spells, if the magi they're creating would have access to it, so on and so forth. And then they'd have to determine which versions have been superseded by advances in Magic Theory Integrations, etc. So probably it isn't worth the trouble.
THAT BEING SAID - it'd be a nice web-research project to through and write up, say the last 100 year's worth of Colentes Arcanorum, with just that information in it.
(As a simplification, you could assume that there is a lessening of interest the further you go back - as Breakthrough Research spells no longer apply, as their Breakthrough has already been integrated.)
EDIT - personally, I just HR that a magus can know a couple of experimental spells that their paren taught them - subject to reasonable storyguide limitations, each paren probably knows one or two that they'd be willing to teach. Beyond that, it's required Storyteller permission and whatnot.
EDIT II - I seem to recall contributing to at least one thread that was doing this last year... (here we go: https://forum.atlas-games.com/t/lets-design-a-folio-updated/8300/1