These are bizarre growths, and it's hard to tell what happened to cause them, but our theory based on how they look is that the trees developed some adventitious growth (roots or shoots, we can't tell at this stage of degradation) from those points on the trunk, which then died. The development of adventitious growths is not uncommon after a major stress, which includes the type of "topping" pruning the trees received, which it sounds like you are aware is very bad for tree health and never recommended as a pruning practice. Complicating the diagnosis is that the growths look distorted and even more crowded than usual, which suggests exposure to an herbicide at some point around the time (or just before) they were developing. Was any weed killer applied at that time? Either way, the condition would not be treatable, even if the plant were still alive.

If a tree doesn't leaf-out by spring or early summer, it's either dead or so weakened that it's probably not worth keeping. The various stressors of its growing conditions (what sounds like limited root space, whether it got watered during drought, and the repeated drastic pruning) contributed to its demise. Even  If they are replaced, we recommend a species other than crapemyrtle, as it is over-planted and non-native, but if new crapemyrtles are used, try to select cultivars that fit the space when mature without needing to be pruned down to size. There are well over a hundred cultivars on the market these days, so there are fortunately plenty of choices. A native species of flowering tree or shrub would provide more wildlife value, though, if that's of interest.

Miri