Compatibility widened to 140.0 – 153.* after verifying the add-on on Thunderbird 153.0.1 as well as Betterbird 140.13.0esr. No code changes were needed — every element ID, item-id, class, <key> and tab mode it relies on still exists in 153, and a real run there confirmed the spaces buttons get hidden, the shortcuts get disabled, the Tools menu entry appears and the prefs are applied.
Install: download the .xpi below, then Add-ons and Themes → gear → Install Add-on From File. Thunderbird does not require signed add-ons on any channel. Then open Tools → Space Control….
Heads up on the Release channel: Thunderbird intends to stop honouring Experiment APIs on Release, gated by extensions.experiments.suppressed. It is false in shipped 153.0.1, so this works today; the switch now targets the 2027 ESR. When it flips, this add-on keeps working on ESR, Beta, Daily and Betterbird, but not on Release.