I had a disc tab welded onto mine about the same time (with brace)
Mods made in period are appropriate as they keep the bike working and updated.
Once a bike is collectable though, its a different thing.
Also mods like this generally reduce collectable value, but increase usability.
You makes your choice.
Welding a brace to the Chainstay would protect the seatstay from some forces, and increase stiffness under braking - but the extra weld runs a risk of weakening the tube, so there's a reason hope might have thought to keep it simple -
It hasn't broken yet, unlike many vitamintys
Mine is still going, sometimes out with modern stuff.
It keeps up better than me.
Brakes loads better than on the original diacompe 857/m900 setup, and better than the xtr and proshift vs it had after.
Also doesn't have wishbone, which was to stiffen up the seatstays to act against rimbrake forces iirc but in turn increases stress on the seat cluster and side to side flex in the rear triangle.
probably insignificantly, although quite a few of these frames crack in the seat cluster.
Love it.