There is a difference between being late to a deadline and being ineligible for it. The rooms where narrative projects in development actually meet money have started writing the tape into the form. Sometimes the tape is the entire point of the program. No footage, no application. Not a weaker application. No application.
I think this is the single most useful thing I know that nobody seems to say out loud, so here it is with the wording copied straight off the forms.
Vimeo link to your original proof of concept video… 2–5 minutes in length. Frontières Proof of Concept, call for projects, Marché du Film
This is the flagship. Frontières is the genre co-production market that runs at Fantasia and at the Marché du Film at Cannes, and it has a section literally named Proof of Concept. Note the word "original": borrowed footage does not qualify. Projects also need to be genre, horror through dark thriller, with at least twenty percent of their financing confirmed. The deliverable is not the script. The deliverable is the video.
Only accepts projects composed of a completed narrative short film with an accompanying completed feature-length screenplay. The Gotham, U.S. Shorts to Features track
The Gotham, formerly IFP, runs a track built entirely on this mechanism: filmmakers with a proof-of-concept short meet executives interested in the feature-length version. The short is not attached to the application. The short is the application. One manager who brokers these deals put it plainly: the short film was the IP for the feature film.
The opportunity to include a link to a directorial sample and/or a PDF of visual materials (e.g., a lookbook or project deck), although this is not required. Sundance Feature Film Program, application FAQ
The softest gate on this list, and read it the way a competitor would. "Not required" in a room this competitive means the people you are up against brought one anyway.
The viability and quality of the plans, and the demonstrable talent of the directors and/or producers. IFFR CineMart, selection criteria
No minute count anywhere on the form. But "demonstrable" is doing loud work in that sentence. In practice the demonstration is moving image, and everyone at that market knows it.
Videos are not required to launch, but projects that have compelling videos tend to succeed at a much higher rate. Kickstarter, creator handbook
Seed&Spark says the same thing in its own guidelines: a campaign page includes a compelling pitch video. If your route to production runs through a campaign instead of a market, the tape is still the toll.
Work the deadlines backwards and the year has a shape. Frontières at Cannes closes in early spring. CineMart closes in early autumn. A sizzle takes three to six weeks if the material exists and longer if it does not, which puts commissioning a full season ahead of the market you are aiming at, not the month before.
Which means the useful question is never "should I make one." It is "which room am I walking into, and when does its form close." Answer that and the rest is arithmetic.
My favourite piece of evidence that this is now standard is not a market at all. It is what the institutions have started paying for. The Proof of Concept Accelerator funds proof-of-concept shorts at fifty thousand dollars each. Screen NSW runs a program named Short to Feature Fast Track that funds, in its own guidelines, a short made as:
a proof of concept for a planned feature film. Screen NSW, Short to Feature Fast Track program guidelines
Public money, in writing, commissioning the exact object this whole page is about. When the money itself starts naming it, the argument is over.