Right now, most topics automatically lock after 14 days for, I dunno, moderation purposes, or something. This limitation does not exist for Gitlab Issues, and Discourse can be a more informal and less fraught way to discuss issues than Gitlab.
In general this short of an automatic timeout seems counterproductive in terms of boosting engagement given the level of activity on this particular Discourse instance. While I imagine you have a strong historical reason for auto-locking old topics, could you experiment with a significantly longer timeout (or disabling the timeout entirely)?
I dunno, like 6 months or something seems both long enough and short enough to match people’s software-update cycles while avoiding resurrecting significantly outdated threads. The timeout could be shorter for certain boards than others, and the timeout could also be shortened for especially busy topics, if Discourse supports doing so.