8. Let $G$ be the point such that $B C D G$ is a parallelogram and let $H$ be the midpoint of $A G$. Obviously $H E F D$ is also a parallelogram, and thus $D H = E F = l$. If $A D^{2} + B C^{2} = m^{2}$ is fixed, then from the Stewart theorem we have
$$D H^{2} = \frac{2 D A^{2} + 2 D G^{2} - A G^{2}}{4} = \frac{2 m^{2} - A G^{2}}{4}$$
which is fixed. Thus $G$ and $H$ are fixed points, and from here the locus of $D$ is a circle with center $H$ and radius $l$. The locus of $B$ is the segment $[GI]$, where $I \in \Delta$ is a point in the positive direction such that $A I = a$. Finally, the locus of $C$ is a region of the plane consisting of a rectangle sandwiched between two semicircles of radius $l$ centered at points $H$ and $H^{\prime}$, where $H^{\prime}$ is a point such that $\overrightarrow{I H^{\prime}} = \overrightarrow{G H}$.