It seems you are asking about two separate topics:
Chromium (and thus Opera) has been moving many download improvements into the stable browser engine. In very recent versions, this specific flag may have been removed because parallel downloading is now enabled by default or handled intelligently by the browser's download manager without needing a flag. If you search opera://flags and cannot find the entry, it means the feature is either permanently on, permanently off, or managed by a different internal system.
Here is a detailed explanation of the feature in Opera, including what it does, how to enable it, and what to expect. opera //flags/enable-parallel-downloading
The "Parallel downloading" flag should be highlighted at the top.
Opera hides experimental features behind a "Flags" menu. Enabling the parallel downloading feature is safe, stable, and takes less than a minute. It seems you are asking about two separate
changes this behavior. When enabled, the browser splits a file into smaller parts and downloads them simultaneously via multiple connections, finally stitching them back together once the download is complete.
You will see a large search bar at the top of the Experiments page. Type: Parallel downloading Here is a detailed explanation of the feature
Launch the browser on your desktop or Android device.
A prompt will appear at the bottom of the screen. Click Relaunch to apply the changes. 💡 Key Benefits Faster Speeds: Maximizes your available bandwidth.