Minimum sizing is disabled in the new docking system primarily because there’s not a good way to preserve the sizing in a shared docking region. We don’t want to scroll the panels because then there would be doubled up scrollbars everywhere.
All that said, I’ve asked the dev team to look into bringing back the minimum sizes for standalone, floating panels and possibly even panels that don’t share the region with other panels. No ETA yet but I think it’s feasible.