If possible, avoid washing old color negatives in plain water. A purpose-made film cleaner is generally the safer first choice for removing dirt from older film. One community recommendation was a cine-film cleaner such as Filmrenew.

If you do wash the negatives and can’t get an old-style stabilizer, a modern wetting agent such as Photo-Flo or an equivalent is the practical substitute mentioned. Older formaldehyde-based stabilizers were discontinued years ago; modern products typically use wetting agents plus mild anti-fungal protection rather than formalin.

Based on the shared answer, the risk of dye migration in these older films is likely low, since the gelatin hardens with age. So for scanning, the priority is gentle cleaning and careful drying rather than trying to replicate obsolete chemistry.

In short: use a dedicated film cleaner if you can; if you must rinse, follow with Photo-Flo/equivalent rather than hunting for discontinued formalin stabilizer.