As long as I've been into cars (26 years) it's always been a continuous weld. You can do multiple small welds as long as they all join up.
Plug welds are acceptable where spot welds were originally. There's two front "jacking panels" on my Golf which are a significant part of the front structure and they are spot welded on by design so plug welding to replace.
I've done a significant amount of work to my Golf, plug weld where spot welds were, continuous seam where a panel is repaired and it's just gone through MOT with no advisories.
I didn't put any underseal on at all, it's in Epox Primer at the moment so the tester could see all repairs (most of the lower half of the car!). He was happy it was all safe and to a good standard.
There's a LOT of areas on a car that are within 30cm of a structural component, engine mounts/ strut tops/ subframe mounts etc.
90 or 91 eh? Makes sense. I'm repairing a 100E (have been putting videos up on YouTube) and it has not been on the road since 1982, so the repairs predate the intro of the continuous welding mandate. It's amazing how shoddy some of the work was. I imagine some of the patches pre dated mig.
Thanks for the info.