Some old stocks cannot be directly processed in contemporary E6 color reversal chemistry. For instance, old Ektachrome (VNF or RNVP processes), Perutz, Agfachrome, Svemacolor, 3M etc. deserve special care. In black & white too, old emulsions have to be dealt with in an appropriate way.
Some labs nonetheless offer the processing of such films.
In Europe, Super 8 lab & Color Film Archives or Film Rescue & Rocky Mountain Film Lab in the USA.
Also there is Dagie Brundert in Germany and Cine-Mechanics in Russia.
Don’t dream of an image exactly as it was meant to be at the time but it is usually possible to see what has been filmed.
In particular, the very specific process meant for Kodachrome isn’t around anywhere, but these films can be processed as black and white negatives and that’s what Film Rescue offers.