In the Davinci Resolve user manual, it states:
In addition, the “trim unused media” options of the Copy or Move operations are now compatible with clips that use codecs employing temporal compression, such as H.264, XAVC, and AVC-Intra, enabling you to eliminate unused media for these formats during media management without recompressing or transcoding.
However, when I actually use this option with H.264 MP4 files, it does NOT keep the same edits that I have made. In the resulting timeline, the clips are all shifted to completely different scenes, and some of the clips even show as offline.
I nearly lost my entire project, had I not been wary and created a backup before attempting this.
If trimming isn't supported for MP4, just say so instead of claiming it works in the manual!
This is Davinci Resolve 17.2.2 Build 4 on fully updated Windows 10.