NO plant likes root congestion. There are times when root congestion might serve the grower because it has an effect on the plant the grower likes, but that isn't a clear indicator the plant will like it. Tight roots cause stress and all sorts of physiological problems for all plants. It reduces growth, branch extension, and leaf size. It limits vitality and caused the shedding of older leaves proximal (closer to the trunk) to growing branch tips (apices). If anyone tells you that a particular plant prefers to be root bound w/o explaining what they really mean is some growers like to maintain a particular plant in root bound conditions, they're wrong. If "some plants" really did prefer root bound conditions, Mother Nature would surely have arranged the plant's genetics so it grew tight little masses of roots right under the stem ..... but we know there are no plants that naturally occur that way. That said, your plant probably isn't in any immediate danger that pertains to root congestion. You're better to suffer the limitations the plant is now growing under for a while in exchange for a more opportune time to repot, which for you would be next June. Plants have natural rhythms, during which their energy levels wax and wane. For major work, like repotting (which includes root pruning), it's best to ask the plant to endure the added stress of repotting or rejuvenation pruning when its energy levels are at peak - not now when they are at or approaching their lowest levels of the entire growth cycle. If you haven't read it, the link below should be very helpful. If you are wondering about repotting and how to maintain trees in containers for the long haul, this link offers lots of guidance. Al Here is a link that might be useful: Lots more info

Why? Are you trying to 'shrink' it? Are you going to move it? Is it in the way and you're not going to move it? We need more info.

In the top picture there is a narrow 'V' where the two main branches diverge. Narrow crotches like this split easier than branches that go out at a wider angle. Snow and ice usually do it as the branches get larger. Kids and dogs are also culprits, as is mishandling. If you look closely this has already happened at the very top of the maple. Choose a maple with a good structure that will hold a good canopy in the future. The lower left limb on my maple has no future as it will eventually be superceded by the branches above it. It does not contribute to supporting the canopy. It has subsequently been removed. I should have spotted it earlier.

Surely you know better the behavior of dicot seedlings than the field of botany. This is, as you said, basic botany. Start a maple from seed, if you don't believe me (or the botanists). Not a grafted rootstock, a seed. Muck about with the roots, observe what they do. They're not as significant as oaks, but maples are dicots and they do have tap roots as seedlings and juveniles. These tap roots give way to the normal surface roots we know and (in bonsai) cultivate. The shoot doesn't have a tap root, just a root that'll be a big bother and should be dealt with using the same sorts of techniques for much the same reasons. Heck, it's even using the root like a tuber. The volunteer seedling evidently does have a tap root, which makes sense because the thing is probably only a couple of years old at the most. It hasn't developed its adventitious roots yet. If it was in grass, that may be a factor in it.