What's the reason for the partition retrofitting note? Is this commonly done for that device?

Flashing the super_empty.img partition is supported, if thats what required.

To be honest, I don't understand everything around retrofit partitions, and it seems to require some additional steps / special recovery... For now, I prefer to support this phone only with “standard” ROMs, and I will look dipper on retrofit partitions when I will have more time. (This applies to some others phones too)

Yes, I see several ROMs for this phone with retrofit partitions, so I write this note just to be sure...

The installation instructions for lineageos 18 don't mention any need for retrofitting: https://wiki.lineageos.org/devices/jasmine_sprout/install/#
Are there more recent roms for this device that might need it?

Looks good to go. Should I merge it?

Yep, I think it's good