Make Closed lost reason useful: check its field type, then require it

Short answer

Closed lost reason is a default deal property (with a matching Closed won reason). In my demo portal it is a multi-line text field, which is hard to group in reports. HubSpot's KB links to customizing its options, and validation rules can apply to it. To make reps fill it in, add a conditional stage rule on the closed lost stage. Note that a rejected deal approval can only move to closed lost.