Back to the question.... I don't know how many chains I've replaced. Certainly hundreds. I estimate 75% of the folks that come into our Co-op need a new chain. Some of these are so egregiously worn out that they are stretched past the ability of the tools we use to measure stretch. Of the worst chains, most have caused accelerated drivetrain wear that have killed the freewheel/cassette and the rings. So a complete drivetrain update required.

Anyway, this very old chain made in the 5-speed era. Bushing based construction that predates the Sedis bushingless design. Primitive plate design, which was much improved by Shimano in their UG, IG and HG chains. So this chain will be laterally stiff and feature slow clunky shifting between cogs.

Measure the chain stretch, and if it fails, toss it and upgrade to a current 5/6 speed derailleur chain. The modern chains shift worlds better than the old ones. Personally, for the improvement in performance, I would just toss the old chain proactively, regardless of the stretch.

These old chains feature pins that extend beyond the plates, so it is generally OK to push a pin (mostly) out to cut links and reconnect. Then push back in. If you've ovalized or oversized the hole in the plates, then you've got a problem, which might solved by peening.

Another suggestion: don't use any water-based degreasers on chains, no matter what they are mixed with or augmented by. Water, being a polar molecule, has almost no ability to dissolve hydrocarbons (grease and oils etc.), it is simply an ineffective degreaser. Further, water is poison to most things on a bike, due to corrosion. Apart from irresponsibly worn-out chains and drivetrains, the other bane of our existence as mechanics is folks who regularly misguidedly 'wash' and 'clean' their bikes with a hose or worse a pressure washer. Every bearing is corroded out and the chain is stiff and damaged beyond saving.

Mineral spirits is far more effective at cleaning chains. Of course, don't use mineral spirits to 'wash' your bike. A damp rag for the outside.