I can't say for sure that they won't mature at all after cutting them off, but I never do that. Every seed saving site I've found advises letting them mature "on the vine" until the pod is good and brown and dry - as close to cracking open as practical.
Right now I have some fresh, immature Columbine vulgaris seeds trying to germinate on a wet coffee filter. I took the brownest pod, which seemed to have a gap starting to appear, "to see how far along the seeds were".
Naaah. The seeds were thin and light tan (immature), not plump, black and shiny (mature). So far, zero germination.
Would they have matured more, if I had left them in the pod for a few weeks? I didn't think so, but I don't really know. When Ia sked the same question a year or two ago on another website, the most enocuraging answer was "you could try it - but if you want mature seeds, let them mature on the plant".
I suppose the question is, how connected are the seeds to the plant's vascular system? Are they still getting nourishment and water, or is the plant just holding them up off the ground so they don't rot before the seed coat hardens?
My assumption is that a plant would scatter its seeds as soon as possible, to avoid giving birds a big, fat free lunch.
If you're afraid of the seeds popping out before you catch them, you can buy some "organza bags" from a craft store or a "wedding supply" isle. (Apparently at wedding receptions, some people give away party favors in gauzy bags with drawstrings.) The organdy gauze lets air through and the drawstring keeps the seeds from getting away.
(Or plop some paper plates under the plants to catch some of the falling seeds, and check it twice per day. Once a few seeds start showing up, tip as many seed heads as you can into a big paper bag and shake out as many loose seeds as you can. Repeat that daily, to catch as many mature seeds as you can.)
That said, I have uprooted aging snow pea plants and not bothered to collect the pods because the peas didn't seem fully mature. Thrown on the compost heap, some % still sprout.
I'll be watching with interest to see if anyone answers "I've done that and gotten X germination". Or "I saved a bunch of pods that had skinny brown seeds at first, that later became fat and black".
